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Active Support Core Extensions

Active Support is the Ruby on Rails component responsible for providing Ruby language extensions, utilities, and other transversal stuff.

It offers a richer bottom-line at the language level, targeted both at the development of Rails applications, and at the development of Ruby on Rails itself.

After reading this guide, you will know:

1 How to Load Core Extensions

1.1 Stand-Alone Active Support

In order to have a near-zero default footprint, Active Support does not load anything by default. It is broken in small pieces so that you can load just what you need, and also has some convenience entry points to load related extensions in one shot, even everything.

Thus, after a simple require like:

require 'active_support'

objects do not even respond to blank?. Let's see how to load its definition.

1.1.1 Cherry-picking a Definition

The most lightweight way to get blank? is to cherry-pick the file that defines it.

For every single method defined as a core extension this guide has a note that says where such a method is defined. In the case of blank? the note reads:

Defined in active_support/core_ext/object/blank.rb.

That means that this single call is enough:

require 'active_support/core_ext/object/blank'

Active Support has been carefully revised so that cherry-picking a file loads only strictly needed dependencies, if any.

1.1.2 Loading Grouped Core Extensions

The next level is to simply load all extensions to Object. As a rule of thumb, extensions to SomeClass are available in one shot by loading active_support/core_ext/some_class.

Thus, to load all extensions to Object (including blank?):

require 'active_support/core_ext/object'

1.1.3 Loading All Core Extensions

You may prefer just to load all core extensions, there is a file for that:

require 'active_support/core_ext'

1.1.4 Loading All Active Support

And finally, if you want to have all Active Support available just issue:

require 'active_support/all'

That does not even put the entire Active Support in memory upfront indeed, some stuff is configured via autoload, so it is only loaded if used.

1.2 Active Support Within a Ruby on Rails Application

A Ruby on Rails application loads all Active Support unless config.active_support.bare is true. In that case, the application will only load what the framework itself cherry-picks for its own needs, and can still cherry-pick itself at any granularity level, as explained in the previous section.

2 Extensions to All Objects

2.1 blank? and present?

The following values are considered to be blank in a Rails application:

  • nil and false,

  • strings composed only of whitespace (see note below),

  • empty arrays and hashes, and

  • any other object that responds to empty? and is empty.

The predicate for strings uses the Unicode-aware character class [:space:], so for example U+2029 (paragraph separator) is considered to be whitespace.

Note that numbers are not mentioned. In particular, 0 and 0.0 are not blank.

For example, this method from ActionDispatch::Session::AbstractStore uses blank? for checking whether a session key is present:

def ensure_session_key!
  if @key.blank?
    raise ArgumentError, 'A key is required...'

The method present? is equivalent to !blank?. This example is taken from ActionDispatch::Http::Cache::Response:

def set_conditional_cache_control!
  return if self["Cache-Control"].present?

Defined in active_support/core_ext/object/blank.rb.

2.2 presence

The presence method returns its receiver if present?, and nil otherwise. It is useful for idioms like this:

host = config[:host].presence || 'localhost'

Defined in active_support/core_ext/object/blank.rb.

2.3 duplicable?

A few fundamental objects in Ruby are singletons. For example, in the whole life of a program the integer 1 refers always to the same instance:

1.object_id                 # => 3
Math.cos(0).to_i.object_id  # => 3

Hence, there's no way these objects can be duplicated through dup or clone:

true.dup  # => TypeError: can't dup TrueClass

Some numbers which are not singletons are not duplicable either:

0.0.clone        # => allocator undefined for Float
(2**1024).clone  # => allocator undefined for Bignum

Active Support provides duplicable? to programmatically query an object about this property:

"foo".duplicable? # => true
"".duplicable?     # => true
0.0.duplicable?   # => false
false.duplicable?  # => false

By definition all objects are duplicable? except nil, false, true, symbols, numbers, class, and module objects.

Any class can disallow duplication by removing dup and clone or raising exceptions from them. Thus only rescue can tell whether a given arbitrary object is duplicable. duplicable? depends on the hard-coded list above, but it is much faster than rescue. Use it only if you know the hard-coded list is enough in your use case.

Defined in active_support/core_ext/object/duplicable.rb.

2.4 deep_dup

The deep_dup method returns deep copy of a given object. Normally, when you dup an object that contains other objects, ruby does not dup them, so it creates a shallow copy of the object. If you have an array with a string, for example, it will look like this:

array     = ['string']
duplicate = array.dup

duplicate.push 'another-string'

# the object was duplicated, so the element was added only to the duplicate
array     #=> ['string']
duplicate #=> ['string', 'another-string']

duplicate.first.gsub!('string', 'foo')

# first element was not duplicated, it will be changed in both arrays
array     #=> ['foo']
duplicate #=> ['foo', 'another-string']

As you can see, after duplicating the Array instance, we got another object, therefore we can modify it and the original object will stay unchanged. This is not true for array's elements, however. Since dup does not make deep copy, the string inside the array is still the same object.

If you need a deep copy of an object, you should use deep_dup. Here is an example:

array     = ['string']
duplicate = array.deep_dup

duplicate.first.gsub!('string', 'foo')

array     #=> ['string']
duplicate #=> ['foo']

If the object is not duplicable, deep_dup will just return it:

number = 1
duplicate = number.deep_dup
number.object_id == duplicate.object_id   # => true

Defined in active_support/core_ext/object/deep_dup.rb.

2.5 try

When you want to call a method on an object only if it is not nil, the simplest way to achieve it is with conditional statements, adding unnecessary clutter. The alternative is to use try. try is like Object#send except that it returns nil if sent to nil.

Here is an example:

# without try
unless @number.nil?

# with try

Another example is this code from ActiveRecord::ConnectionAdapters::AbstractAdapter where @logger could be nil. You can see that the code uses try and avoids an unnecessary check.

def log_info(sql, name, ms)
  if @logger.try(:debug?)
    name = '%s (%.1fms)' % [name || 'SQL', ms]
    @logger.debug(format_log_entry(name, sql.squeeze(' ')))

try can also be called without arguments but a block, which will only be executed if the object is not nil:

@person.try { |p| "#{p.first_name} #{p.last_name}" }

Defined in active_support/core_ext/object/try.rb.

2.6 class_eval(*args, &block)

You can evaluate code in the context of any object's singleton class using class_eval:

class Proc
  def bind(object)
    block, time = self, Time.current
    object.class_eval do
      method_name = "__bind_#{time.to_i}_#{time.usec}"
      define_method(method_name, &block)
      method = instance_method(method_name)

Defined in active_support/core_ext/kernel/singleton_class.rb.

2.7 acts_like?(duck)

The method acts_like? provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as String defines

def acts_like_string?

which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:


Rails has classes that act like Date or Time and follow this contract.

Defined in active_support/core_ext/object/acts_like.rb.

2.8 to_param

All objects in Rails respond to the method to_param, which is meant to return something that represents them as values in a query string, or as URL fragments.

By default to_param just calls to_s:

7.to_param # => "7"

The return value of to_param should not be escaped:

"Tom & Jerry".to_param # => "Tom & Jerry"

Several classes in Rails overwrite this method.

For example nil, true, and false return themselves. Array#to_param calls to_param on the elements and joins the result with "/":

[0, true, String].to_param # => "0/true/String"

Notably, the Rails routing system calls to_param on models to get a value for the :id placeholder. ActiveRecord::Base#to_param returns the id of a model, but you can redefine that method in your models. For example, given

class User
  def to_param

we get:

user_path(@user) # => "/users/357-john-smith"

Controllers need to be aware of any redefinition of to_param because when a request like that comes in "357-john-smith" is the value of params[:id].

Defined in active_support/core_ext/object/to_param.rb.

2.9 to_query

Except for hashes, given an unescaped key this method constructs the part of a query string that would map such key to what to_param returns. For example, given

class User
  def to_param

we get:

current_user.to_query('user') # => user=357-john-smith

This method escapes whatever is needed, both for the key and the value:

# => "company%5Bname%5D=Johnson+%26+Johnson"

so its output is ready to be used in a query string.

Arrays return the result of applying to_query to each element with _key_[] as key, and join the result with "&":

[3.4, -45.6].to_query('sample')
# => "sample%5B%5D=3.4&sample%5B%5D=-45.6"

Hashes also respond to to_query but with a different signature. If no argument is passed a call generates a sorted series of key/value assignments calling to_query(key) on its values. Then it joins the result with "&":

{c: 3, b: 2, a: 1}.to_query # => "a=1&b=2&c=3"

The method Hash#to_query accepts an optional namespace for the keys:

{id: 89, name: "John Smith"}.to_query('user')
# => "user%5Bid%5D=89&user%5Bname%5D=John+Smith"

Defined in active_support/core_ext/object/to_query.rb.

2.10 with_options

The method with_options provides a way to factor out common options in a series of method calls.

Given a default options hash, with_options yields a proxy object to a block. Within the block, methods called on the proxy are forwarded to the receiver with their options merged. For example, you get rid of the duplication in:

class Account < ActiveRecord::Base
  has_many :customers, dependent: :destroy
  has_many :products,  dependent: :destroy
  has_many :invoices,  dependent: :destroy
  has_many :expenses,  dependent: :destroy

this way:

class Account < ActiveRecord::Base
  with_options dependent: :destroy do |assoc|
    assoc.has_many :customers
    assoc.has_many :products
    assoc.has_many :invoices
    assoc.has_many :expenses

That idiom may convey grouping to the reader as well. For example, say you want to send a newsletter whose language depends on the user. Somewhere in the mailer you could group locale-dependent bits like this:

I18n.with_options locale: user.locale, scope: "newsletter" do |i18n|
  subject i18n.t :subject
  body    i18n.t :body, user_name:

Since with_options forwards calls to its receiver they can be nested. Each nesting level will merge inherited defaults in addition to their own.

Defined in active_support/core_ext/object/with_options.rb.

2.11 JSON support

Active Support provides a better implemention of to_json than the +json+ gem ordinarily provides for Ruby objects. This is because some classes, like +Hash+ and +OrderedHash+ needs special handling in order to provide a proper JSON representation.

Active Support also provides an implementation of as_json for the Process::Status class.

Defined in active_support/core_ext/object/to_json.rb.

2.12 Instance Variables

Active Support provides several methods to ease access to instance variables.

2.12.1 instance_values

The method instance_values returns a hash that maps instance variable names without "@" to their corresponding values. Keys are strings:

class C
  def initialize(x, y)
    @x, @y = x, y
end, 1).instance_values # => {"x" => 0, "y" => 1}

Defined in active_support/core_ext/object/instance_variables.rb.

2.12.2 instance_variable_names

The method instance_variable_names returns an array. Each name includes the "@" sign.

class C
  def initialize(x, y)
    @x, @y = x, y
end, 1).instance_variable_names # => ["@x", "@y"]

Defined in active_support/core_ext/object/instance_variables.rb.

2.13 Silencing Warnings, Streams, and Exceptions

The methods silence_warnings and enable_warnings change the value of $VERBOSE accordingly for the duration of their block, and reset it afterwards:

silence_warnings { Object.const_set "RAILS_DEFAULT_LOGGER", logger }

You can silence any stream while a block runs with silence_stream:

silence_stream(STDOUT) do
  # STDOUT is silent here

The quietly method addresses the common use case where you want to silence STDOUT and STDERR, even in subprocesses:

quietly { system 'bundle install' }

For example, the railties test suite uses that one in a few places to prevent command messages from being echoed intermixed with the progress status.

Silencing exceptions is also possible with suppress. This method receives an arbitrary number of exception classes. If an exception is raised during the execution of the block and is kind_of? any of the arguments, suppress captures it and returns silently. Otherwise the exception is reraised:

# If the user is locked the increment is lost, no big deal.
suppress(ActiveRecord::StaleObjectError) do
  current_user.increment! :visits

Defined in active_support/core_ext/kernel/reporting.rb.

2.14 in?

The predicate in? tests if an object is included in another object. An ArgumentError exception will be raised if the argument passed does not respond to include?.

Examples of in?:[1,2])        # => true
"lo".in?("hello")   # => true      # => false            # => ArgumentError

Defined in active_support/core_ext/object/inclusion.rb.

3 Extensions to Module

3.1 alias_method_chain

Using plain Ruby you can wrap methods with other methods, that's called alias chaining.

For example, let's say you'd like params to be strings in functional tests, as they are in real requests, but still want the convenience of assigning integers and other kind of values. To accomplish that you could wrap ActionController::TestCase#process this way in test/test_helper.rb:

ActionController::TestCase.class_eval do
  # save a reference to the original process method
  alias_method :original_process, :process

  # now redefine process and delegate to original_process
  def process(action, params=nil, session=nil, flash=nil, http_method='GET')
    params = Hash[* {|k, v| [k, v.to_s]}.flatten]
    original_process(action, params, session, flash, http_method)

That's the method get, post, etc., delegate the work to.

That technique has a risk, it could be the case that :original_process was taken. To try to avoid collisions people choose some label that characterizes what the chaining is about:

ActionController::TestCase.class_eval do
  def process_with_stringified_params(...)
    params = Hash[* {|k, v| [k, v.to_s]}.flatten]
    process_without_stringified_params(action, params, session, flash, http_method)
  alias_method :process_without_stringified_params, :process
  alias_method :process, :process_with_stringified_params

The method alias_method_chain provides a shortcut for that pattern:

ActionController::TestCase.class_eval do
  def process_with_stringified_params(...)
    params = Hash[* {|k, v| [k, v.to_s]}.flatten]
    process_without_stringified_params(action, params, session, flash, http_method)
  alias_method_chain :process, :stringified_params

Rails uses alias_method_chain all over the code base. For example validations are added to ActiveRecord::Base#save by wrapping the method that way in a separate module specialized in validations.

Defined in active_support/core_ext/module/aliasing.rb.

3.2 Attributes

3.2.1 alias_attribute

Model attributes have a reader, a writer, and a predicate. You can alias a model attribute having the corresponding three methods defined for you in one shot. As in other aliasing methods, the new name is the first argument, and the old name is the second (my mnemonic is they go in the same order as if you did an assignment):

class User < ActiveRecord::Base
  # let me refer to the email column as "login",
  # possibly meaningful for authentication code
  alias_attribute :login, :email

Defined in active_support/core_ext/module/aliasing.rb.

3.2.2 Internal Attributes

When you are defining an attribute in a class that is meant to be subclassed, name collisions are a risk. That's remarkably important for libraries.

Active Support defines the macros attr_internal_reader, attr_internal_writer, and attr_internal_accessor. They behave like their Ruby built-in attr_* counterparts, except they name the underlying instance variable in a way that makes collisions less likely.

The macro attr_internal is a synonym for attr_internal_accessor:

# library
class ThirdPartyLibrary::Crawler
  attr_internal :log_level

# client code
class MyCrawler < ThirdPartyLibrary::Crawler
  attr_accessor :log_level

In the previous example it could be the case that :log_level does not belong to the public interface of the library and it is only used for development. The client code, unaware of the potential conflict, subclasses and defines its own :log_level. Thanks to attr_internal there's no collision.

By default the internal instance variable is named with a leading underscore, @_log_level in the example above. That's configurable via Module.attr_internal_naming_format though, you can pass any sprintf-like format string with a leading @ and a %s somewhere, which is where the name will be placed. The default is "@_%s".

Rails uses internal attributes in a few spots, for examples for views:

module ActionView
  class Base
    attr_internal :captures
    attr_internal :request, :layout
    attr_internal :controller, :template

Defined in active_support/core_ext/module/attr_internal.rb.

3.2.3 Module Attributes

The macros mattr_reader, mattr_writer, and mattr_accessor are analogous to the cattr_* macros defined for class. Check Class Attributes.

For example, the dependencies mechanism uses them:

module ActiveSupport
  module Dependencies
    mattr_accessor :warnings_on_first_load
    mattr_accessor :history
    mattr_accessor :loaded
    mattr_accessor :mechanism
    mattr_accessor :load_paths
    mattr_accessor :load_once_paths
    mattr_accessor :autoloaded_constants
    mattr_accessor :explicitly_unloadable_constants
    mattr_accessor :logger
    mattr_accessor :log_activity
    mattr_accessor :constant_watch_stack
    mattr_accessor :constant_watch_stack_mutex

Defined in active_support/core_ext/module/attribute_accessors.rb.

3.3 Parents

3.3.1 parent

The parent method on a nested named module returns the module that contains its corresponding constant:

module X
  module Y
    module Z
M = X::Y::Z

X::Y::Z.parent # => X::Y
M.parent       # => X::Y

If the module is anonymous or belongs to the top-level, parent returns Object.

Note that in that case parent_name returns nil.

Defined in active_support/core_ext/module/introspection.rb.

3.3.2 parent_name

The parent_name method on a nested named module returns the fully-qualified name of the module that contains its corresponding constant:

module X
  module Y
    module Z
M = X::Y::Z

X::Y::Z.parent_name # => "X::Y"
M.parent_name       # => "X::Y"

For top-level or anonymous modules parent_name returns nil.

Note that in that case parent returns Object.

Defined in active_support/core_ext/module/introspection.rb.

3.3.3 parents

The method parents calls parent on the receiver and upwards until Object is reached. The chain is returned in an array, from bottom to top:

module X
  module Y
    module Z
M = X::Y::Z

X::Y::Z.parents # => [X::Y, X, Object]
M.parents       # => [X::Y, X, Object]

Defined in active_support/core_ext/module/introspection.rb.

3.4 Constants

The method local_constants returns the names of the constants that have been defined in the receiver module:

module X
  X1 = 1
  X2 = 2
  module Y
    Y1 = :y1
    X1 = :overrides_X1_above

X.local_constants    # => [:X1, :X2, :Y]
X::Y.local_constants # => [:Y1, :X1]

The names are returned as symbols. (The deprecated method local_constant_names returns strings.)

Defined in active_support/core_ext/module/introspection.rb.

3.4.1 Qualified Constant Names

The standard methods const_defined?, const_get , and const_set accept bare constant names. Active Support extends this API to be able to pass relative qualified constant names.

The new methods are qualified_const_defined?, qualified_const_get, and qualified_const_set. Their arguments are assumed to be qualified constant names relative to their receiver:

Object.qualified_const_defined?("Math::PI")       # => true
Object.qualified_const_get("Math::PI")            # => 3.141592653589793
Object.qualified_const_set("Math::Phi", 1.618034) # => 1.618034

Arguments may be bare constant names:

Math.qualified_const_get("E") # => 2.718281828459045

These methods are analogous to their builtin counterparts. In particular, qualified_constant_defined? accepts an optional second argument to be able to say whether you want the predicate to look in the ancestors. This flag is taken into account for each constant in the expression while walking down the path.

For example, given

module M
  X = 1

module N
  class C
    include M

qualified_const_defined? behaves this way:

N.qualified_const_defined?("C::X", false) # => false
N.qualified_const_defined?("C::X", true)  # => true
N.qualified_const_defined?("C::X")        # => true

As the last example implies, the second argument defaults to true, as in const_defined?.

For coherence with the builtin methods only relative paths are accepted. Absolute qualified constant names like ::Math::PI raise NameError.

Defined in active_support/core_ext/module/qualified_const.rb.

3.5 Reachable

A named module is reachable if it is stored in its corresponding constant. It means you can reach the module object via the constant.

That is what ordinarily happens, if a module is called "M", the M constant exists and holds it:

module M

M.reachable? # => true

But since constants and modules are indeed kind of decoupled, module objects can become unreachable:

module M

orphan = Object.send(:remove_const, :M)

# The module object is orphan now but it still has a name. # => "M"

# You cannot reach it via the constant M because it does not even exist.
orphan.reachable? # => false

# Let's define a module called "M" again.
module M

# The constant M exists now again, and it stores a module
# object called "M", but it is a new instance.
orphan.reachable? # => false

Defined in active_support/core_ext/module/reachable.rb.

3.6 Anonymous

A module may or may not have a name:

module M
end # => "M"

N = # => "N" # => nil

You can check whether a module has a name with the predicate anonymous?:

module M
M.anonymous? # => false # => true

Note that being unreachable does not imply being anonymous:

module M

m = Object.send(:remove_const, :M)

m.reachable? # => false
m.anonymous? # => false

though an anonymous module is unreachable by definition.

Defined in active_support/core_ext/module/anonymous.rb.

3.7 Method Delegation

The macro delegate offers an easy way to forward methods.

Let's imagine that users in some application have login information in the User model but name and other data in a separate Profile model:

class User < ActiveRecord::Base
  has_one :profile

With that configuration you get a user's name via their profile,, but it could be handy to still be able to access such attribute directly:

class User < ActiveRecord::Base
  has_one :profile

  def name

That is what delegate does for you:

class User < ActiveRecord::Base
  has_one :profile

  delegate :name, to: :profile

It is shorter, and the intention more obvious.

The method must be public in the target.

The delegate macro accepts several methods:

delegate :name, :age, :address, :twitter, to: :profile

When interpolated into a string, the :to option should become an expression that evaluates to the object the method is delegated to. Typically a string or symbol. Such an expression is evaluated in the context of the receiver:

# delegates to the Rails constant
delegate :logger, to: :Rails

# delegates to the receiver's class
delegate :table_name, to: :class

If the :prefix option is true this is less generic, see below.

By default, if the delegation raises NoMethodError and the target is nil the exception is propagated. You can ask that nil is returned instead with the :allow_nil option:

delegate :name, to: :profile, allow_nil: true

With :allow_nil the call returns nil if the user has no profile.

The option :prefix adds a prefix to the name of the generated method. This may be handy for example to get a better name:

delegate :street, to: :address, prefix: true

The previous example generates address_street rather than street.

Since in this case the name of the generated method is composed of the target object and target method names, the :to option must be a method name.

A custom prefix may also be configured:

delegate :size, to: :attachment, prefix: :avatar

In the previous example the macro generates avatar_size rather than size.

Defined in active_support/core_ext/module/delegation.rb

3.8 Redefining Methods

There are cases where you need to define a method with define_method, but don't know whether a method with that name already exists. If it does, a warning is issued if they are enabled. No big deal, but not clean either.

The method redefine_method prevents such a potential warning, removing the existing method before if needed. Rails uses it in a few places, for instance when it generates an association's API:

redefine_method("#{}=") do |new_value|
  association = association_instance_get(

  if association.nil? || != new_value
    association =, reflection)

  association_instance_set(, new_value.nil? ? nil : association)

Defined in active_support/core_ext/module/remove_method.rb

4 Extensions to Class

4.1 Class Attributes

4.1.1 class_attribute

The method class_attribute declares one or more inheritable class attributes that can be overridden at any level down the hierarchy.

class A
  class_attribute :x

class B < A; end

class C < B; end

A.x = :a
B.x # => :a
C.x # => :a

B.x = :b
A.x # => :a
C.x # => :b

C.x = :c
A.x # => :a
B.x # => :b

For example ActionMailer::Base defines:

class_attribute :default_params
self.default_params = {
  mime_version: "1.0",
  charset: "UTF-8",
  content_type: "text/plain",
  parts_order: [ "text/plain", "text/enriched", "text/html" ]

They can be also accessed and overridden at the instance level.

A.x = 1

a1 =
a2 =
a2.x = 2

a1.x # => 1, comes from A
a2.x # => 2, overridden in a2

The generation of the writer instance method can be prevented by setting the option :instance_writer to false.

module ActiveRecord
  class Base
    class_attribute :table_name_prefix, instance_writer: false
    self.table_name_prefix = ""

A model may find that option useful as a way to prevent mass-assignment from setting the attribute.

The generation of the reader instance method can be prevented by setting the option :instance_reader to false.

class A
  class_attribute :x, instance_reader: false
end = 1 # NoMethodError

For convenience class_attribute also defines an instance predicate which is the double negation of what the instance reader returns. In the examples above it would be called x?.

When :instance_reader is false, the instance predicate returns a NoMethodError just like the reader method.

If you do not want the instance predicate, pass instance_predicate: false and it will not be defined.

Defined in active_support/core_ext/class/attribute.rb

4.1.2 cattr_reader, cattr_writer, and cattr_accessor

The macros cattr_reader, cattr_writer, and cattr_accessor are analogous to their attr_* counterparts but for classes. They initialize a class variable to nil unless it already exists, and generate the corresponding class methods to access it:

class MysqlAdapter < AbstractAdapter
  # Generates class methods to access @@emulate_booleans.
  cattr_accessor :emulate_booleans
  self.emulate_booleans = true

Instance methods are created as well for convenience, they are just proxies to the class attribute. So, instances can change the class attribute, but cannot override it as it happens with class_attribute (see above). For example given

module ActionView
  class Base
    cattr_accessor :field_error_proc
    @@field_error_proc ={ ... }

we can access field_error_proc in views.

The generation of the reader instance method can be prevented by setting :instance_reader to false and the generation of the writer instance method can be prevented by setting :instance_writer to false. Generation of both methods can be prevented by setting :instance_accessor to false. In all cases, the value must be exactly false and not any false value.

module A
  class B
    # No first_name instance reader is generated.
    cattr_accessor :first_name, instance_reader: false
    # No last_name= instance writer is generated.
    cattr_accessor :last_name, instance_writer: false
    # No surname instance reader or surname= writer is generated.
    cattr_accessor :surname, instance_accessor: false

A model may find it useful to set :instance_accessor to false as a way to prevent mass-assignment from setting the attribute.

Defined in active_support/core_ext/class/attribute_accessors.rb.

4.2 Subclasses & Descendants

4.2.1 subclasses

The subclasses method returns the subclasses of the receiver:

class C; end
C.subclasses # => []

class B < C; end
C.subclasses # => [B]

class A < B; end
C.subclasses # => [B]

class D < C; end
C.subclasses # => [B, D]

The order in which these classes are returned is unspecified.

Defined in active_support/core_ext/class/subclasses.rb.

4.2.2 descendants

The descendants method returns all classes that are < than its receiver:

class C; end
C.descendants # => []

class B < C; end
C.descendants # => [B]

class A < B; end
C.descendants # => [B, A]

class D < C; end
C.descendants # => [B, A, D]

The order in which these classes are returned is unspecified.

Defined in active_support/core_ext/class/subclasses.rb.

5 Extensions to String

5.1 Output Safety

5.1.1 Motivation

Inserting data into HTML templates needs extra care. For example, you can't just interpolate @review.title verbatim into an HTML page. For one thing, if the review title is "Flanagan & Matz rules!" the output won't be well-formed because an ampersand has to be escaped as "&amp;". What's more, depending on the application, that may be a big security hole because users can inject malicious HTML setting a hand-crafted review title. Check out the section about cross-site scripting in the Security guide for further information about the risks.

5.1.2 Safe Strings

Active Support has the concept of (html) safe strings. A safe string is one that is marked as being insertable into HTML as is. It is trusted, no matter whether it has been escaped or not.

Strings are considered to be unsafe by default:

"".html_safe? # => false

You can obtain a safe string from a given one with the html_safe method:

s = "".html_safe
s.html_safe? # => true

It is important to understand that html_safe performs no escaping whatsoever, it is just an assertion:

s = "<script>...</script>".html_safe
s.html_safe? # => true
s            # => "<script>...</script>"

It is your responsibility to ensure calling html_safe on a particular string is fine.

If you append onto a safe string, either in-place with concat/<<, or with +, the result is a safe string. Unsafe arguments are escaped:

"".html_safe + "<" # => "&lt;"

Safe arguments are directly appended:

"".html_safe + "<".html_safe # => "<"

These methods should not be used in ordinary views. Unsafe values are automatically escaped:

<%= @review.title %> <%# fine, escaped if needed %>

To insert something verbatim use the raw helper rather than calling html_safe:

<%= raw @cms.current_template %> <%# inserts @cms.current_template as is %>

or, equivalently, use <%==:

<%== @cms.current_template %> <%# inserts @cms.current_template as is %>

The raw helper calls html_safe for you:

def raw(stringish)

Defined in active_support/core_ext/string/output_safety.rb.

5.1.3 Transformation

As a rule of thumb, except perhaps for concatenation as explained above, any method that may change a string gives you an unsafe string. These are downcase, gsub, strip, chomp, underscore, etc.

In the case of in-place transformations like gsub! the receiver itself becomes unsafe.

The safety bit is lost always, no matter whether the transformation actually changed something.

5.1.4 Conversion and Coercion

Calling to_s on a safe string returns a safe string, but coercion with to_str returns an unsafe string.

5.1.5 Copying

Calling dup or clone on safe strings yields safe strings.

5.2 squish

The method squish strips leading and trailing whitespace, and substitutes runs of whitespace with a single space each:

" \n  foo\n\r \t bar \n".squish # => "foo bar"

There's also the destructive version String#squish!.

Note that it handles both ASCII and Unicode whitespace like mongolian vowel separator (U+180E).

Defined in active_support/core_ext/string/filters.rb.

5.3 truncate

The method truncate returns a copy of its receiver truncated after a given length:

"Oh dear! Oh dear! I shall be late!".truncate(20)
# => "Oh dear! Oh dear!..."

Ellipsis can be customized with the :omission option:

"Oh dear! Oh dear! I shall be late!".truncate(20, omission: '&hellip;')
# => "Oh dear! Oh &hellip;"

Note in particular that truncation takes into account the length of the omission string.

Pass a :separator to truncate the string at a natural break:

"Oh dear! Oh dear! I shall be late!".truncate(18)
# => "Oh dear! Oh dea..."
"Oh dear! Oh dear! I shall be late!".truncate(18, separator: ' ')
# => "Oh dear! Oh..."

The option :separator can be a regexp:

"Oh dear! Oh dear! I shall be late!".truncate(18, separator: /\s/)
# => "Oh dear! Oh..."

In above examples "dear" gets cut first, but then :separator prevents it.

Defined in active_support/core_ext/string/filters.rb.

5.4 inquiry

The inquiry method converts a string into a StringInquirer object making equality checks prettier.

"production".inquiry.production? # => true
"active".inquiry.inactive?       # => false

5.5 starts_with? and ends_with?

Active Support defines 3rd person aliases of String#start_with? and String#end_with?:

"foo".starts_with?("f") # => true
"foo".ends_with?("o")   # => true

Defined in active_support/core_ext/string/starts_ends_with.rb.

5.6 strip_heredoc

The method strip_heredoc strips indentation in heredocs.

For example in

if options[:usage]
  puts <<-USAGE.strip_heredoc
    This command does such and such.

    Supported options are:
      -h         This message

the user would see the usage message aligned against the left margin.

Technically, it looks for the least indented line in the whole string, and removes that amount of leading whitespace.

Defined in active_support/core_ext/string/strip.rb.

5.7 indent

Indents the lines in the receiver:

def some_method
# =>
  def some_method

The second argument, indent_string, specifies which indent string to use. The default is nil, which tells the method to make an educated guess peeking at the first indented line, and fallback to a space if there is none.

"  foo".indent(2)        # => "    foo"
"foo\n\t\tbar".indent(2) # => "\t\tfoo\n\t\t\t\tbar"
"foo".indent(2, "\t")    # => "\t\tfoo"

While indent_string is typically one space or tab, it may be any string.

The third argument, indent_empty_lines, is a flag that says whether empty lines should be indented. Default is false.

"foo\n\nbar".indent(2)            # => "  foo\n\n  bar"
"foo\n\nbar".indent(2, nil, true) # => "  foo\n  \n  bar"

The indent! method performs indentation in-place.

Defined in active_support/core_ext/string/indent.rb.

5.8 Access

5.8.1 at(position)

Returns the character of the string at position position:

"hello".at(0)  # => "h"
"hello".at(4)  # => "o"
"hello".at(-1) # => "o"
"hello".at(10) # => nil

Defined in active_support/core_ext/string/access.rb.

5.8.2 from(position)

Returns the substring of the string starting at position position:

"hello".from(0)  # => "hello"
"hello".from(2)  # => "llo"
"hello".from(-2) # => "lo"
"hello".from(10) # => "" if < 1.9, nil in 1.9

Defined in active_support/core_ext/string/access.rb.

5.8.3 to(position)

Returns the substring of the string up to position position:

"hello".to(0)  # => "h"
"hello".to(2)  # => "hel"
"hello".to(-2) # => "hell"
"hello".to(10) # => "hello"

Defined in active_support/core_ext/string/access.rb.

5.8.4 first(limit = 1)

The call str.first(n) is equivalent to if n > 0, and returns an empty string for n == 0.

Defined in active_support/core_ext/string/access.rb.

5.8.5 last(limit = 1)

The call str.last(n) is equivalent to str.from(-n) if n > 0, and returns an empty string for n == 0.

Defined in active_support/core_ext/string/access.rb.

5.9 Inflections

5.9.1 pluralize

The method pluralize returns the plural of its receiver:

"table".pluralize     # => "tables"
"ruby".pluralize      # => "rubies"
"equipment".pluralize # => "equipment"

As the previous example shows, Active Support knows some irregular plurals and uncountable nouns. Built-in rules can be extended in config/initializers/inflections.rb. That file is generated by the rails command and has instructions in comments.

pluralize can also take an optional count parameter. If count == 1 the singular form will be returned. For any other value of count the plural form will be returned:

"dude".pluralize(0) # => "dudes"
"dude".pluralize(1) # => "dude"
"dude".pluralize(2) # => "dudes"

Active Record uses this method to compute the default table name that corresponds to a model:

# active_record/model_schema.rb
def undecorated_table_name(class_name =
  table_name = class_name.to_s.demodulize.underscore
  pluralize_table_names ? table_name.pluralize : table_name

Defined in active_support/core_ext/string/inflections.rb.

5.9.2 singularize

The inverse of pluralize:

"tables".singularize    # => "table"
"rubies".singularize    # => "ruby"
"equipment".singularize # => "equipment"

Associations compute the name of the corresponding default associated class using this method:

# active_record/reflection.rb
def derive_class_name
  class_name = name.to_s.camelize
  class_name = class_name.singularize if collection?

Defined in active_support/core_ext/string/inflections.rb.

5.9.3 camelize

The method camelize returns its receiver in camel case:

"product".camelize    # => "Product"
"admin_user".camelize # => "AdminUser"

As a rule of thumb you can think of this method as the one that transforms paths into Ruby class or module names, where slashes separate namespaces:

"backoffice/session".camelize # => "Backoffice::Session"

For example, Action Pack uses this method to load the class that provides a certain session store:

# action_controller/metal/session_management.rb
def session_store=(store)
  @@session_store = store.is_a?(Symbol) ?
    ActionDispatch::Session.const_get(store.to_s.camelize) :

camelize accepts an optional argument, it can be :upper (default), or :lower. With the latter the first letter becomes lowercase:

"visual_effect".camelize(:lower) # => "visualEffect"

That may be handy to compute method names in a language that follows that convention, for example JavaScript.

As a rule of thumb you can think of camelize as the inverse of underscore, though there are cases where that does not hold: "SSLError".underscore.camelize gives back "SslError". To support cases such as this, Active Support allows you to specify acronyms in config/initializers/inflections.rb:

ActiveSupport::Inflector.inflections do |inflect|
  inflect.acronym 'SSL'

"SSLError".underscore.camelize #=> "SSLError"

camelize is aliased to camelcase.

Defined in active_support/core_ext/string/inflections.rb.

5.9.4 underscore

The method underscore goes the other way around, from camel case to paths:

"Product".underscore   # => "product"
"AdminUser".underscore # => "admin_user"

Also converts "::" back to "/":

"Backoffice::Session".underscore # => "backoffice/session"

and understands strings that start with lowercase:

"visualEffect".underscore # => "visual_effect"

underscore accepts no argument though.

Rails class and module autoloading uses underscore to infer the relative path without extension of a file that would define a given missing constant:

# active_support/dependencies.rb
def load_missing_constant(from_mod, const_name)
  qualified_name = qualified_name_for from_mod, const_name
  path_suffix = qualified_name.underscore

As a rule of thumb you can think of underscore as the inverse of camelize, though there are cases where that does not hold. For example, "SSLError".underscore.camelize gives back "SslError".

Defined in active_support/core_ext/string/inflections.rb.

5.9.5 titleize

The method titleize capitalizes the words in the receiver:

"alice in wonderland".titleize # => "Alice In Wonderland"
"fermat's enigma".titleize     # => "Fermat's Enigma"

titleize is aliased to titlecase.

Defined in active_support/core_ext/string/inflections.rb.

5.9.6 dasherize

The method dasherize replaces the underscores in the receiver with dashes:

"name".dasherize         # => "name"
"contact_data".dasherize # => "contact-data"

The XML serializer of models uses this method to dasherize node names:

# active_model/serializers/xml.rb
def reformat_name(name)
  name = name.camelize if camelize?
  dasherize? ? name.dasherize : name

Defined in active_support/core_ext/string/inflections.rb.

5.9.7 demodulize

Given a string with a qualified constant name, demodulize returns the very constant name, that is, the rightmost part of it:

"Product".demodulize                        # => "Product"
"Backoffice::UsersController".demodulize    # => "UsersController"
"Admin::Hotel::ReservationUtils".demodulize # => "ReservationUtils"

Active Record for example uses this method to compute the name of a counter cache column:

# active_record/reflection.rb
def counter_cache_column
  if options[:counter_cache] == true
  elsif options[:counter_cache]

Defined in active_support/core_ext/string/inflections.rb.

5.9.8 deconstantize

Given a string with a qualified constant reference expression, deconstantize removes the rightmost segment, generally leaving the name of the constant's container:

"Product".deconstantize                        # => ""
"Backoffice::UsersController".deconstantize    # => "Backoffice"
"Admin::Hotel::ReservationUtils".deconstantize # => "Admin::Hotel"

Active Support for example uses this method in Module#qualified_const_set:

def qualified_const_set(path, value)

  const_name = path.demodulize
  mod_name = path.deconstantize
  mod = mod_name.empty? ? self : qualified_const_get(mod_name)
  mod.const_set(const_name, value)

Defined in active_support/core_ext/string/inflections.rb.

5.9.9 parameterize

The method parameterize normalizes its receiver in a way that can be used in pretty URLs.

"John Smith".parameterize # => "john-smith"
"Kurt Gödel".parameterize # => "kurt-godel"

In fact, the result string is wrapped in an instance of ActiveSupport::Multibyte::Chars.

Defined in active_support/core_ext/string/inflections.rb.

5.9.10 tableize

The method tableize is underscore followed by pluralize.

"Person".tableize      # => "people"
"Invoice".tableize     # => "invoices"
"InvoiceLine".tableize # => "invoice_lines"

As a rule of thumb, tableize returns the table name that corresponds to a given model for simple cases. The actual implementation in Active Record is not straight tableize indeed, because it also demodulizes the class name and checks a few options that may affect the returned string.

Defined in active_support/core_ext/string/inflections.rb.

5.9.11 classify

The method classify is the inverse of tableize. It gives you the class name corresponding to a table name:

"people".classify        # => "Person"
"invoices".classify      # => "Invoice"
"invoice_lines".classify # => "InvoiceLine"

The method understands qualified table names:

"highrise_production.companies".classify # => "Company"

Note that classify returns a class name as a string. You can get the actual class object invoking constantize on it, explained next.

Defined in active_support/core_ext/string/inflections.rb.

5.9.12 constantize

The method constantize resolves the constant reference expression in its receiver:

"Fixnum".constantize # => Fixnum

module M
  X = 1
"M::X".constantize # => 1

If the string evaluates to no known constant, or its content is not even a valid constant name, constantize raises NameError.

Constant name resolution by constantize starts always at the top-level Object even if there is no leading "::".

X = :in_Object
module M
  X = :in_M

  X                 # => :in_M
  "::X".constantize # => :in_Object
  "X".constantize   # => :in_Object (!)

So, it is in general not equivalent to what Ruby would do in the same spot, had a real constant be evaluated.

Mailer test cases obtain the mailer being tested from the name of the test class using constantize:

# action_mailer/test_case.rb
def determine_default_mailer(name)
  name.sub(/Test$/, '').constantize
rescue NameError => e

Defined in active_support/core_ext/string/inflections.rb.

5.9.13 humanize

The method humanize gives you a sensible name for display out of an attribute name. To do so it replaces underscores with spaces, removes any "_id" suffix, and capitalizes the first word:

"name".humanize           # => "Name"
"author_id".humanize      # => "Author"
"comments_count".humanize # => "Comments count"

The helper method full_messages uses humanize as a fallback to include attribute names:

def full_messages
  full_messages = []

  each do |attribute, messages|
    attr_name = attribute.to_s.gsub('.', '_').humanize
    attr_name = @base.class.human_attribute_name(attribute, default: attr_name)


Defined in active_support/core_ext/string/inflections.rb.

5.9.14 foreign_key

The method foreign_key gives a foreign key column name from a class name. To do so it demodulizes, underscores, and adds "_id":

"User".foreign_key           # => "user_id"
"InvoiceLine".foreign_key    # => "invoice_line_id"
"Admin::Session".foreign_key # => "session_id"

Pass a false argument if you do not want the underscore in "_id":

"User".foreign_key(false) # => "userid"

Associations use this method to infer foreign keys, for example has_one and has_many do this:

# active_record/associations.rb
foreign_key = options[:foreign_key] ||

Defined in active_support/core_ext/string/inflections.rb.

5.10 Conversions

5.10.1 to_date, to_time, to_datetime

The methods to_date, to_time, and to_datetime are basically convenience wrappers around Date._parse:

"2010-07-27".to_date              # => Tue, 27 Jul 2010
"2010-07-27 23:37:00".to_time     # => Tue Jul 27 23:37:00 UTC 2010
"2010-07-27 23:37:00".to_datetime # => Tue, 27 Jul 2010 23:37:00 +0000

to_time receives an optional argument :utc or :local, to indicate which time zone you want the time in:

"2010-07-27 23:42:00".to_time(:utc)   # => Tue Jul 27 23:42:00 UTC 2010
"2010-07-27 23:42:00".to_time(:local) # => Tue Jul 27 23:42:00 +0200 2010

Default is :utc.

Please refer to the documentation of Date._parse for further details.

The three of them return nil for blank receivers.

Defined in active_support/core_ext/string/conversions.rb.

6 Extensions to Numeric

6.1 Bytes

All numbers respond to these methods:


They return the corresponding amount of bytes, using a conversion factor of 1024:

2.kilobytes   # => 2048
3.megabytes   # => 3145728
3.5.gigabytes # => 3758096384
-4.exabytes   # => -4611686018427387904

Singular forms are aliased so you are able to say:

1.megabyte # => 1048576

Defined in active_support/core_ext/numeric/bytes.rb.

6.2 Time

Enables the use of time calculations and declarations, like 45.minutes + 2.hours + 4.years.

These methods use Time#advance for precise date calculations when using from_now, ago, etc. as well as adding or subtracting their results from a Time object. For example:

# equivalent to Time.current.advance(months: 1)

# equivalent to Time.current.advance(years: 2)

# equivalent to Time.current.advance(months: 4, years: 5)
(4.months + 5.years).from_now

While these methods provide precise calculation when used as in the examples above, care should be taken to note that this is not true if the result of months',years', etc is converted before use:

# equivalent to 30.days.to_i.from_now

# equivalent to 365.25.days.to_f.from_now

In such cases, Ruby's core Date and Time should be used for precision date and time arithmetic.

Defined in active_support/core_ext/numeric/time.rb.

6.3 Formatting

Enables the formatting of numbers in a variety of ways.

Produce a string representation of a number as a telephone number:

# => 555-1234
# => 123-555-1234
1235551234.to_s(:phone, area_code: true)
# => (123) 555-1234
1235551234.to_s(:phone, delimiter: " ")
# => 123 555 1234
1235551234.to_s(:phone, area_code: true, extension: 555)
# => (123) 555-1234 x 555
1235551234.to_s(:phone, country_code: 1)
# => +1-123-555-1234

Produce a string representation of a number as currency:

1234567890.50.to_s(:currency)                 # => $1,234,567,890.50
1234567890.506.to_s(:currency)                # => $1,234,567,890.51
1234567890.506.to_s(:currency, precision: 3)  # => $1,234,567,890.506

Produce a string representation of a number as a percentage:

# => 100.000%
100.to_s(:percentage, precision: 0)
# => 100%
1000.to_s(:percentage, delimiter: '.', separator: ',')
# => 1.000,000%
302.24398923423.to_s(:percentage, precision: 5)
# => 302.24399%

Produce a string representation of a number in delimited form:

12345678.to_s(:delimited)                     # => 12,345,678
12345678.05.to_s(:delimited)                  # => 12,345,678.05
12345678.to_s(:delimited, delimiter: ".")     # => 12.345.678
12345678.to_s(:delimited, delimiter: ",")     # => 12,345,678
12345678.05.to_s(:delimited, separator: " ")  # => 12,345,678 05

Produce a string representation of a number rounded to a precision:

111.2345.to_s(:rounded)                     # => 111.235
111.2345.to_s(:rounded, precision: 2)       # => 111.23
13.to_s(:rounded, precision: 5)             # => 13.00000
389.32314.to_s(:rounded, precision: 0)      # => 389
111.2345.to_s(:rounded, significant: true)  # => 111

Produce a string representation of a number as a human-readable number of bytes:

123.to_s(:human_size)            # => 123 Bytes
1234.to_s(:human_size)           # => 1.21 KB
12345.to_s(:human_size)          # => 12.1 KB
1234567.to_s(:human_size)        # => 1.18 MB
1234567890.to_s(:human_size)     # => 1.15 GB
1234567890123.to_s(:human_size)  # => 1.12 TB

Produce a string representation of a number in human-readable words:

123.to_s(:human)               # => "123"
1234.to_s(:human)              # => "1.23 Thousand"
12345.to_s(:human)             # => "12.3 Thousand"
1234567.to_s(:human)           # => "1.23 Million"
1234567890.to_s(:human)        # => "1.23 Billion"
1234567890123.to_s(:human)     # => "1.23 Trillion"
1234567890123456.to_s(:human)  # => "1.23 Quadrillion"

Defined in active_support/core_ext/numeric/formatting.rb.

7 Extensions to Integer

7.1 multiple_of?

The method multiple_of? tests whether an integer is multiple of the argument:

2.multiple_of?(1) # => true
1.multiple_of?(2) # => false

Defined in active_support/core_ext/integer/multiple.rb.

7.2 ordinal

The method ordinal returns the ordinal suffix string corresponding to the receiver integer:

1.ordinal    # => "st"
2.ordinal    # => "nd"
53.ordinal   # => "rd"
2009.ordinal # => "th"
-21.ordinal  # => "st"
-134.ordinal # => "th"

Defined in active_support/core_ext/integer/inflections.rb.

7.3 ordinalize

The method ordinalize returns the ordinal string corresponding to the receiver integer. In comparison, note that the ordinal method returns only the suffix string.

1.ordinalize    # => "1st"
2.ordinalize    # => "2nd"
53.ordinalize   # => "53rd"
2009.ordinalize # => "2009th"
-21.ordinalize  # => "-21st"
-134.ordinalize # => "-134th"

Defined in active_support/core_ext/integer/inflections.rb.

8 Extensions to BigDecimal

8.1 to_s

The method to_s is aliased to to_formatted_s. This provides a convenient way to display a BigDecimal value in floating-point notation:, 6).to_s  # => "5.0"

8.2 to_formatted_s

Te method to_formatted_s provides a default specifier of "F". This means that a simple call to to_formatted_s or to_s will result in floating point representation instead of engineering notation:, 6).to_formatted_s  # => "5.0"

and that symbol specifiers are also supported:, 6).to_formatted_s(:db)  # => "5.0"

Engineering notation is still supported:, 6).to_formatted_s("e")  # => "0.5E1"

9 Extensions to Enumerable

9.1 sum

The method sum adds the elements of an enumerable:

[1, 2, 3].sum # => 6
(1..100).sum  # => 5050

Addition only assumes the elements respond to +:

[[1, 2], [2, 3], [3, 4]].sum    # => [1, 2, 2, 3, 3, 4]
%w(foo bar baz).sum             # => "foobarbaz"
{a: 1, b: 2, c: 3}.sum # => [:b, 2, :c, 3, :a, 1]

The sum of an empty collection is zero by default, but this is customizable:

[].sum    # => 0
[].sum(1) # => 1

If a block is given, sum becomes an iterator that yields the elements of the collection and sums the returned values:

(1..5).sum {|n| n * 2 } # => 30
[2, 4, 6, 8, 10].sum    # => 30

The sum of an empty receiver can be customized in this form as well:

[].sum(1) {|n| n**3} # => 1

Defined in active_support/core_ext/enumerable.rb.

9.2 index_by

The method index_by generates a hash with the elements of an enumerable indexed by some key.

It iterates through the collection and passes each element to a block. The element will be keyed by the value returned by the block:

# => {'2009-032' => <Invoice ...>, '2009-008' => <Invoice ...>, ...}

Keys should normally be unique. If the block returns the same value for different elements no collection is built for that key. The last item will win.

Defined in active_support/core_ext/enumerable.rb.

9.3 many?

The method many? is shorthand for collection.size > 1:

<% if pages.many? %>
  <%= pagination_links %>
<% end %>

If an optional block is given, many? only takes into account those elements that return true:

@see_more = videos.many? {|video| video.category == params[:category]}

Defined in active_support/core_ext/enumerable.rb.

9.4 exclude?

The predicate exclude? tests whether a given object does not belong to the collection. It is the negation of the built-in include?:

to_visit << node if visited.exclude?(node)

Defined in active_support/core_ext/enumerable.rb.

10 Extensions to Array

10.1 Accessing

Active Support augments the API of arrays to ease certain ways of accessing them. For example, to returns the subarray of elements up to the one at the passed index:

%w(a b c d).to(2) # => %w(a b c)
[].to(7)          # => []

Similarly, from returns the tail from the element at the passed index to the end. If the index is greater than the length of the array, it returns an empty array.

%w(a b c d).from(2)  # => %w(c d)
%w(a b c d).from(10) # => []
[].from(0)           # => []

The methods second, third, fourth, and fifth return the corresponding element (first is built-in). Thanks to social wisdom and positive constructiveness all around, forty_two is also available.

%w(a b c d).third # => c
%w(a b c d).fifth # => nil

Defined in active_support/core_ext/array/access.rb.

10.2 Adding Elements

10.2.1 prepend

This method is an alias of Array#unshift.

%w(a b c d).prepend('e')  # => %w(e a b c d)
[].prepend(10)            # => [10]

Defined in active_support/core_ext/array/prepend_and_append.rb.

10.2.2 append

This method is an alias of Array#<<.

%w(a b c d).append('e')  # => %w(a b c d e)
[].append([1,2])         # => [[1,2]]

Defined in active_support/core_ext/array/prepend_and_append.rb.

10.3 Options Extraction

When the last argument in a method call is a hash, except perhaps for a &block argument, Ruby allows you to omit the brackets:

User.exists?(email: params[:email])

That syntactic sugar is used a lot in Rails to avoid positional arguments where there would be too many, offering instead interfaces that emulate named parameters. In particular it is very idiomatic to use a trailing hash for options.

If a method expects a variable number of arguments and uses * in its declaration, however, such an options hash ends up being an item of the array of arguments, where it loses its role.

In those cases, you may give an options hash a distinguished treatment with extract_options!. This method checks the type of the last item of an array. If it is a hash it pops it and returns it, otherwise it returns an empty hash.

Let's see for example the definition of the caches_action controller macro:

def caches_action(*actions)
  return unless cache_configured?
  options = actions.extract_options!

This method receives an arbitrary number of action names, and an optional hash of options as last argument. With the call to extract_options! you obtain the options hash and remove it from actions in a simple and explicit way.

Defined in active_support/core_ext/array/extract_options.rb.

10.4 Conversions

10.4.1 to_sentence

The method to_sentence turns an array into a string containing a sentence that enumerates its items:

%w().to_sentence                # => ""
%w(Earth).to_sentence           # => "Earth"
%w(Earth Wind).to_sentence      # => "Earth and Wind"
%w(Earth Wind Fire).to_sentence # => "Earth, Wind, and Fire"

This method accepts three options:

  • :two_words_connector: What is used for arrays of length 2. Default is " and ".
  • :words_connector: What is used to join the elements of arrays with 3 or more elements, except for the last two. Default is ", ".
  • :last_word_connector: What is used to join the last items of an array with 3 or more elements. Default is ", and ".

The defaults for these options can be localized, their keys are:

Option I18n key
:two_words_connector support.array.two_words_connector
:words_connector support.array.words_connector
:last_word_connector support.array.last_word_connector

Defined in active_support/core_ext/array/conversions.rb.

10.4.2 to_formatted_s

The method to_formatted_s acts like to_s by default.

If the array contains items that respond to id, however, the symbol :db may be passed as argument. That's typically used with collections of Active Record objects. Returned strings are:

[].to_formatted_s(:db)            # => "null"
[user].to_formatted_s(:db)        # => "8456"
invoice.lines.to_formatted_s(:db) # => "23,567,556,12"

Integers in the example above are supposed to come from the respective calls to id.

Defined in active_support/core_ext/array/conversions.rb.

10.4.3 to_xml

The method to_xml returns a string containing an XML representation of its receiver:

# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <contributors type="array">
#   <contributor>
#     <id type="integer">4356</id>
#     <name>Jeremy Kemper</name>
#     <rank type="integer">1</rank>
#     <url-id>jeremy-kemper</url-id>
#   </contributor>
#   <contributor>
#     <id type="integer">4404</id>
#     <name>David Heinemeier Hansson</name>
#     <rank type="integer">2</rank>
#     <url-id>david-heinemeier-hansson</url-id>
#   </contributor>
# </contributors>

To do so it sends to_xml to every item in turn, and collects the results under a root node. All items must respond to to_xml, an exception is raised otherwise.

By default, the name of the root element is the underscorized and dasherized plural of the name of the class of the first item, provided the rest of elements belong to that type (checked with is_a?) and they are not hashes. In the example above that's "contributors".

If there's any element that does not belong to the type of the first one the root node becomes "objects":

[Contributor.first, Commit.first].to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <objects type="array">
#   <object>
#     <id type="integer">4583</id>
#     <name>Aaron Batalion</name>
#     <rank type="integer">53</rank>
#     <url-id>aaron-batalion</url-id>
#   </object>
#   <object>
#     <author>Joshua Peek</author>
#     <authored-timestamp type="datetime">2009-09-02T16:44:36Z</authored-timestamp>
#     <branch>origin/master</branch>
#     <committed-timestamp type="datetime">2009-09-02T16:44:36Z</committed-timestamp>
#     <committer>Joshua Peek</committer>
#     <git-show nil="true"></git-show>
#     <id type="integer">190316</id>
#     <imported-from-svn type="boolean">false</imported-from-svn>
#     <message>Kill AMo observing wrap_with_notifications since ARes was only using it</message>
#     <sha1>723a47bfb3708f968821bc969a9a3fc873a3ed58</sha1>
#   </object>
# </objects>

If the receiver is an array of hashes the root element is by default also "objects":

[{a: 1, b: 2}, {c: 3}].to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <objects type="array">
#   <object>
#     <b type="integer">2</b>
#     <a type="integer">1</a>
#   </object>
#   <object>
#     <c type="integer">3</c>
#   </object>
# </objects>

If the collection is empty the root element is by default "nil-classes". That's a gotcha, for example the root element of the list of contributors above would not be "contributors" if the collection was empty, but "nil-classes". You may use the :root option to ensure a consistent root element.

The name of children nodes is by default the name of the root node singularized. In the examples above we've seen "contributor" and "object". The option :children allows you to set these node names.

The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder via the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder:

Contributor.limit(2).order(:rank).to_xml(skip_types: true)
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <contributors>
#   <contributor>
#     <id>4356</id>
#     <name>Jeremy Kemper</name>
#     <rank>1</rank>
#     <url-id>jeremy-kemper</url-id>
#   </contributor>
#   <contributor>
#     <id>4404</id>
#     <name>David Heinemeier Hansson</name>
#     <rank>2</rank>
#     <url-id>david-heinemeier-hansson</url-id>
#   </contributor>
# </contributors>

Defined in active_support/core_ext/array/conversions.rb.

10.5 Wrapping

The method Array.wrap wraps its argument in an array unless it is already an array (or array-like).


  • If the argument is nil an empty list is returned.
  • Otherwise, if the argument responds to to_ary it is invoked, and if the value of to_ary is not nil, it is returned.
  • Otherwise, an array with the argument as its single element is returned.
Array.wrap(nil)       # => []
Array.wrap([1, 2, 3]) # => [1, 2, 3]
Array.wrap(0)         # => [0]

This method is similar in purpose to Kernel#Array, but there are some differences:

  • If the argument responds to to_ary the method is invoked. Kernel#Array moves on to try to_a if the returned value is nil, but Array.wrap returns nil right away.
  • If the returned value from to_ary is neither nil nor an Array object, Kernel#Array raises an exception, while Array.wrap does not, it just returns the value.
  • It does not call to_a on the argument, though special-cases nil to return an empty array.

The last point is particularly worth comparing for some enumerables:

Array.wrap(foo: :bar) # => [{:foo=>:bar}]
Array(foo: :bar)      # => [[:foo, :bar]]

There's also a related idiom that uses the splat operator:


which in Ruby 1.8 returns [nil] for nil, and calls to Array(object) otherwise. (Please if you know the exact behavior in 1.9 contact fxn.)

Thus, in this case the behavior is different for nil, and the differences with Kernel#Array explained above apply to the rest of objects.

Defined in active_support/core_ext/array/wrap.rb.

10.6 Duplicating

The method Array.deep_dup duplicates itself and all objects inside recursively with Active Support method Object#deep_dup. It works like Array#map with sending deep_dup method to each object inside.

array = [1, [2, 3]]
dup = array.deep_dup
dup[1][2] = 4
array[1][2] == nil   # => true

Defined in active_support/core_ext/array/deep_dup.rb.

10.7 Grouping

10.7.1 in_groups_of(number, fill_with = nil)

The method in_groups_of splits an array into consecutive groups of a certain size. It returns an array with the groups:

[1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]

or yields them in turn if a block is passed:

<% sample.in_groups_of(3) do |a, b, c| %>
    <td><%= a %></td>
    <td><%= b %></td>
    <td><%= c %></td>
<% end %>

The first example shows in_groups_of fills the last group with as many nil elements as needed to have the requested size. You can change this padding value using the second optional argument:

[1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]

And you can tell the method not to fill the last group passing false:

[1, 2, 3].in_groups_of(2, false) # => [[1, 2], [3]]

As a consequence false can't be a used as a padding value.

Defined in active_support/core_ext/array/grouping.rb.

10.7.2 in_groups(number, fill_with = nil)

The method in_groups splits an array into a certain number of groups. The method returns an array with the groups:

%w(1 2 3 4 5 6 7).in_groups(3)
# => [["1", "2", "3"], ["4", "5", nil], ["6", "7", nil]]

or yields them in turn if a block is passed:

%w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
["1", "2", "3"]
["4", "5", nil]
["6", "7", nil]

The examples above show that in_groups fills some groups with a trailing nil element as needed. A group can get at most one of these extra elements, the rightmost one if any. And the groups that have them are always the last ones.

You can change this padding value using the second optional argument:

%w(1 2 3 4 5 6 7).in_groups(3, "0")
# => [["1", "2", "3"], ["4", "5", "0"], ["6", "7", "0"]]

And you can tell the method not to fill the smaller groups passing false:

%w(1 2 3 4 5 6 7).in_groups(3, false)
# => [["1", "2", "3"], ["4", "5"], ["6", "7"]]

As a consequence false can't be a used as a padding value.

Defined in active_support/core_ext/array/grouping.rb.

10.7.3 split(value = nil)

The method split divides an array by a separator and returns the resulting chunks.

If a block is passed the separators are those elements of the array for which the block returns true:

(-5..5).to_a.split { |i| i.multiple_of?(4) }
# => [[-5], [-3, -2, -1], [1, 2, 3], [5]]

Otherwise, the value received as argument, which defaults to nil, is the separator:

[0, 1, -5, 1, 1, "foo", "bar"].split(1)
# => [[0], [-5], [], ["foo", "bar"]]

Observe in the previous example that consecutive separators result in empty arrays.

Defined in active_support/core_ext/array/grouping.rb.

11 Extensions to Hash

11.1 Conversions

11.1.1 to_xml

The method to_xml returns a string containing an XML representation of its receiver:

{"foo" => 1, "bar" => 2}.to_xml
# =>
# <?xml version="1.0" encoding="UTF-8"?>
# <hash>
#   <foo type="integer">1</foo>
#   <bar type="integer">2</bar>
# </hash>

To do so, the method loops over the pairs and builds nodes that depend on the values. Given a pair key, value:

  • If value is a hash there's a recursive call with key as :root.

  • If value is an array there's a recursive call with key as :root, and key singularized as :children.

  • If value is a callable object it must expect one or two arguments. Depending on the arity, the callable is invoked with the options hash as first argument with key as :root, and key singularized as second argument. Its return value becomes a new node.

  • If value responds to to_xml the method is invoked with key as :root.

  • Otherwise, a node with key as tag is created with a string representation of value as text node. If value is nil an attribute "nil" set to "true" is added. Unless the option :skip_types exists and is true, an attribute "type" is added as well according to the following mapping:

  "Symbol"     => "symbol",
  "Fixnum"     => "integer",
  "Bignum"     => "integer",
  "BigDecimal" => "decimal",
  "Float"      => "float",
  "TrueClass"  => "boolean",
  "FalseClass" => "boolean",
  "Date"       => "date",
  "DateTime"   => "datetime",
  "Time"       => "datetime"

By default the root node is "hash", but that's configurable via the :root option.

The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder with the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder.

Defined in active_support/core_ext/hash/conversions.rb.

11.2 Merging

Ruby has a built-in method Hash#merge that merges two hashes:

{a: 1, b: 1}.merge(a: 0, c: 2)
# => {:a=>0, :b=>1, :c=>2}

Active Support defines a few more ways of merging hashes that may be convenient.

11.2.1 reverse_merge and reverse_merge!

In case of collision the key in the hash of the argument wins in merge. You can support option hashes with default values in a compact way with this idiom:

options = {length: 30, omission: "..."}.merge(options)

Active Support defines reverse_merge in case you prefer this alternative notation:

options = options.reverse_merge(length: 30, omission: "...")

And a bang version reverse_merge! that performs the merge in place:

options.reverse_merge!(length: 30, omission: "...")

Take into account that reverse_merge! may change the hash in the caller, which may or may not be a good idea.

Defined in active_support/core_ext/hash/reverse_merge.rb.

11.2.2 reverse_update

The method reverse_update is an alias for reverse_merge!, explained above.

Note that reverse_update has no bang.

Defined in active_support/core_ext/hash/reverse_merge.rb.

11.2.3 deep_merge and deep_merge!

As you can see in the previous example if a key is found in both hashes the value in the one in the argument wins.

Active Support defines Hash#deep_merge. In a deep merge, if a key is found in both hashes and their values are hashes in turn, then their merge becomes the value in the resulting hash:

{a: {b: 1}}.deep_merge(a: {c: 2})
# => {:a=>{:b=>1, :c=>2}}

The method deep_merge! performs a deep merge in place.

Defined in active_support/core_ext/hash/deep_merge.rb.

11.3 Deep duplicating

The method Hash.deep_dup duplicates itself and all keys and values inside recursively with Active Support method Object#deep_dup. It works like Enumerator#each_with_object with sending deep_dup method to each pair inside.

hash = { a: 1, b: { c: 2, d: [3, 4] } }

dup = hash.deep_dup
dup[:b][:e] = 5
dup[:b][:d] << 5

hash[:b][:e] == nil      # => true
hash[:b][:d] == [3, 4]   # => true

Defined in active_support/core_ext/hash/deep_dup.rb.

11.4 Diffing

The method diff returns a hash that represents a diff of the receiver and the argument with the following logic:

  • Pairs key, value that exist in both hashes do not belong to the diff hash.

  • If both hashes have key, but with different values, the pair in the receiver wins.

  • The rest is just merged.

{a: 1}.diff(a: 1)
# => {}, first rule

{a: 1}.diff(a: 2)
# => {:a=>1}, second rule

{a: 1}.diff(b: 2)
# => {:a=>1, :b=>2}, third rule

{a: 1, b: 2, c: 3}.diff(b: 1, c: 3, d: 4)
# => {:a=>1, :b=>2, :d=>4}, all rules

{}.diff({})        # => {}
{a: 1}.diff({})    # => {:a=>1}
{}.diff(a: 1)      # => {:a=>1}

An important property of this diff hash is that you can retrieve the original hash by applying diff twice:

hash.diff(hash2).diff(hash2) == hash

Diffing hashes may be useful for error messages related to expected option hashes for example.

Defined in active_support/core_ext/hash/diff.rb.

11.5 Working with Keys

11.5.1 except and except!

The method except returns a hash with the keys in the argument list removed, if present:

{a: 1, b: 2}.except(:a) # => {:b=>2}

If the receiver responds to convert_key, the method is called on each of the arguments. This allows except to play nice with hashes with indifferent access for instance:

{a: 1}.with_indifferent_access.except(:a)  # => {}
{a: 1}.with_indifferent_access.except("a") # => {}

There's also the bang variant except! that removes keys in the very receiver.

Defined in active_support/core_ext/hash/except.rb.

11.5.2 transform_keys and transform_keys!

The method transform_keys accepts a block and returns a hash that has applied the block operations to each of the keys in the receiver:

{nil => nil, 1 => 1, a: :a}.transform_keys{ |key| key.to_s.upcase }
# => {"" => nil, "A" => :a, "1" => 1}

The result in case of collision is undefined:

{"a" => 1, a: 2}.transform_keys{ |key| key.to_s.upcase }
# => {"A" => 2}, in my test, can't rely on this result though

This method may be useful for example to build specialized conversions. For instance stringify_keys and symbolize_keys use transform_keys to perform their key conversions:

def stringify_keys
  transform_keys{ |key| key.to_s }
def symbolize_keys
  transform_keys{ |key| key.to_sym rescue key }

There's also the bang variant transform_keys! that applies the block operations to keys in the very receiver.

Besides that, one can use deep_transform_keys and deep_transform_keys! to perform the block operation on all the keys in the given hash and all the hashes nested into it. An example of the result is:

{nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_transform_keys{ |key| key.to_s.upcase }
# => {""=>nil, "1"=>1, "NESTED"=>{"A"=>3, "5"=>5}}

Defined in active_support/core_ext/hash/keys.rb.

11.5.3 stringify_keys and stringify_keys!

The method stringify_keys returns a hash that has a stringified version of the keys in the receiver. It does so by sending to_s to them:

{nil => nil, 1 => 1, a: :a}.stringify_keys
# => {"" => nil, "a" => :a, "1" => 1}

The result in case of collision is undefined:

{"a" => 1, a: 2}.stringify_keys
# => {"a" => 2}, in my test, can't rely on this result though

This method may be useful for example to easily accept both symbols and strings as options. For instance ActionView::Helpers::FormHelper defines:

def to_check_box_tag(options = {}, checked_value = "1", unchecked_value = "0")
  options = options.stringify_keys
  options["type"] = "checkbox"

The second line can safely access the "type" key, and let the user to pass either :type or "type".

There's also the bang variant stringify_keys! that stringifies keys in the very receiver.

Besides that, one can use deep_stringify_keys and deep_stringify_keys! to stringify all the keys in the given hash and all the hashes nested into it. An example of the result is:

{nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_stringify_keys
# => {""=>nil, "1"=>1, "nested"=>{"a"=>3, "5"=>5}}

Defined in active_support/core_ext/hash/keys.rb.

11.5.4 symbolize_keys and symbolize_keys!

The method symbolize_keys returns a hash that has a symbolized version of the keys in the receiver, where possible. It does so by sending to_sym to them:

{nil => nil, 1 => 1, "a" => "a"}.symbolize_keys
# => {1=>1, nil=>nil, :a=>"a"}

Note in the previous example only one key was symbolized.

The result in case of collision is undefined:

{"a" => 1, a: 2}.symbolize_keys
# => {:a=>2}, in my test, can't rely on this result though

This method may be useful for example to easily accept both symbols and strings as options. For instance ActionController::UrlRewriter defines

def rewrite_path(options)
  options = options.symbolize_keys
  options.update(options[:params].symbolize_keys) if options[:params]

The second line can safely access the :params key, and let the user to pass either :params or "params".

There's also the bang variant symbolize_keys! that symbolizes keys in the very receiver.

Besides that, one can use deep_symbolize_keys and deep_symbolize_keys! to symbolize all the keys in the given hash and all the hashes nested into it. An example of the result is:

{nil => nil, 1 => 1, "nested" => {"a" => 3, 5 => 5}}.deep_symbolize_keys
# => {nil=>nil, 1=>1, nested:{a:3, 5=>5}}

Defined in active_support/core_ext/hash/keys.rb.

11.5.5 to_options and to_options!

The methods to_options and to_options! are respectively aliases of symbolize_keys and symbolize_keys!.

Defined in active_support/core_ext/hash/keys.rb.

11.5.6 assert_valid_keys

The method assert_valid_keys receives an arbitrary number of arguments, and checks whether the receiver has any key outside that white list. If it does ArgumentError is raised.

{a: 1}.assert_valid_keys(:a)  # passes
{a: 1}.assert_valid_keys("a") # ArgumentError

Active Record does not accept unknown options when building associations, for example. It implements that control via assert_valid_keys.

Defined in active_support/core_ext/hash/keys.rb.

11.6 Slicing

Ruby has built-in support for taking slices out of strings and arrays. Active Support extends slicing to hashes:

{a: 1, b: 2, c: 3}.slice(:a, :c)
# => {:c=>3, :a=>1}

{a: 1, b: 2, c: 3}.slice(:b, :X)
# => {:b=>2} # non-existing keys are ignored

If the receiver responds to convert_key keys are normalized:

{a: 1, b: 2}.with_indifferent_access.slice("a")
# => {:a=>1}

Slicing may come in handy for sanitizing option hashes with a white list of keys.

There's also slice! which in addition to perform a slice in place returns what's removed:

hash = {a: 1, b: 2}
rest = hash.slice!(:a) # => {:b=>2}
hash                   # => {:a=>1}

Defined in active_support/core_ext/hash/slice.rb.

11.7 Extracting

The method extract! removes and returns the key/value pairs matching the given keys.

hash = {a: 1, b: 2}
rest = hash.extract!(:a) # => {:a=>1}
hash                     # => {:b=>2}

The method extract! returns the same subclass of Hash, that the receiver is.

hash = {a: 1, b: 2}.with_indifferent_access
rest = hash.extract!(:a).class
# => ActiveSupport::HashWithIndifferentAccess

Defined in active_support/core_ext/hash/slice.rb.

11.8 Indifferent Access

The method with_indifferent_access returns an ActiveSupport::HashWithIndifferentAccess out of its receiver:

{a: 1}.with_indifferent_access["a"] # => 1

Defined in active_support/core_ext/hash/indifferent_access.rb.

12 Extensions to Regexp

12.1 multiline?

The method multiline? says whether a regexp has the /m flag set, that is, whether the dot matches newlines.

%r{.}.multiline?  # => false
%r{.}m.multiline? # => true'.').multiline?                    # => false'.', Regexp::MULTILINE).multiline? # => true

Rails uses this method in a single place, also in the routing code. Multiline regexps are disallowed for route requirements and this flag eases enforcing that constraint.

def assign_route_options(segments, defaults, requirements)
  if requirement.multiline?
    raise ArgumentError, "Regexp multiline option not allowed in routing requirements: #{requirement.inspect}"

Defined in active_support/core_ext/regexp.rb.

13 Extensions to Range

13.1 to_s

Active Support extends the method Range#to_s so that it understands an optional format argument. As of this writing the only supported non-default format is :db:

# => "2009-10-25..2009-10-26"

# => "BETWEEN '2009-10-25' AND '2009-10-26'"

As the example depicts, the :db format generates a BETWEEN SQL clause. That is used by Active Record in its support for range values in conditions.

Defined in active_support/core_ext/range/conversions.rb.

13.2 include?

The methods Range#include? and Range#=== say whether some value falls between the ends of a given instance:

(2..3).include?(Math::E) # => true

Active Support extends these methods so that the argument may be another range in turn. In that case we test whether the ends of the argument range belong to the receiver themselves:

(1..10).include?(3..7)  # => true
(1..10).include?(0..7)  # => false
(1..10).include?(3..11) # => false
(1...9).include?(3..9)  # => false

(1..10) === (3..7)  # => true
(1..10) === (0..7)  # => false
(1..10) === (3..11) # => false
(1...9) === (3..9)  # => false

Defined in active_support/core_ext/range/include_range.rb.

13.3 overlaps?

The method Range#overlaps? says whether any two given ranges have non-void intersection:

(1..10).overlaps?(7..11)  # => true
(1..10).overlaps?(0..7)   # => true
(1..10).overlaps?(11..27) # => false

Defined in active_support/core_ext/range/overlaps.rb.

14 Extensions to Proc

14.1 bind

As you surely know Ruby has an UnboundMethod class whose instances are methods that belong to the limbo of methods without a self. The method Module#instance_method returns an unbound method for example:

Hash.instance_method(:delete) # => #<UnboundMethod: Hash#delete>

An unbound method is not callable as is, you need to bind it first to an object with bind:

clear = Hash.instance_method(:clear)
clear.bind({a: 1}).call # => {}

Active Support defines Proc#bind with an analogous purpose: { size }.bind([]).call # => 0

As you see that's callable and bound to the argument, the return value is indeed a Method.

To do so Proc#bind actually creates a method under the hood. If you ever see a method with a weird name like __bind_1256598120_237302 in a stack trace you know now where it comes from.

Action Pack uses this trick in rescue_from for example, which accepts the name of a method and also a proc as callbacks for a given rescued exception. It has to call them in either case, so a bound method is returned by handler_for_rescue, thus simplifying the code in the caller:

def handler_for_rescue(exception)
  _, rescuer = Array(rescue_handlers).reverse.detect do |klass_name, handler|

  case rescuer
  when Symbol
  when Proc

Defined in active_support/core_ext/proc.rb.

15 Extensions to Date

15.1 Calculations

All the following methods are defined in active_support/core_ext/date/calculations.rb.

The following calculation methods have edge cases in October 1582, since days 5..14 just do not exist. This guide does not document their behavior around those days for brevity, but it is enough to say that they do what you would expect. That is,, 10, 4).tomorrow returns, 10, 15) and so on. Please check test/core_ext/date_ext_test.rb in the Active Support test suite for expected behavior.

15.1.1 Date.current

Active Support defines Date.current to be today in the current time zone. That's like, except that it honors the user time zone, if defined. It also defines Date.yesterday and Date.tomorrow, and the instance predicates past?, today?, and future?, all of them relative to Date.current.

When making Date comparisons using methods which honor the user time zone, make sure to use Date.current and not There are cases where the user time zone might be in the future compared to the system time zone, which uses by default. This means may equal Date.yesterday.

15.1.2 Named dates prev_year, next_year

In Ruby 1.9 prev_year and next_year return a date with the same day/month in the last or next year:

d =, 5, 8) # => Sat, 08 May 2010
d.prev_year              # => Fri, 08 May 2009
d.next_year              # => Sun, 08 May 2011

If date is the 29th of February of a leap year, you obtain the 28th:

d =, 2, 29) # => Tue, 29 Feb 2000
d.prev_year               # => Sun, 28 Feb 1999
d.next_year               # => Wed, 28 Feb 2001

prev_year is aliased to last_year. prev_month, next_month

In Ruby 1.9 prev_month and next_month return the date with the same day in the last or next month:

d =, 5, 8) # => Sat, 08 May 2010
d.prev_month             # => Thu, 08 Apr 2010
d.next_month             # => Tue, 08 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:, 5, 31).prev_month # => Sun, 30 Apr 2000, 3, 31).prev_month # => Tue, 29 Feb 2000, 5, 31).next_month # => Fri, 30 Jun 2000, 1, 31).next_month # => Tue, 29 Feb 2000

prev_month is aliased to last_month. prev_quarter, next_quarter

Same as prev_month and next_month. It returns the date with the same day in the previous or next quarter:

t = Time.local(2010, 5, 8) # => Sat, 08 May 2010
t.prev_quarter             # => Mon, 08 Feb 2010
t.next_quarter             # => Sun, 08 Aug 2010

If such a day does not exist, the last day of the corresponding month is returned:

Time.local(2000, 7, 31).prev_quarter  # => Sun, 30 Apr 2000
Time.local(2000, 5, 31).prev_quarter  # => Tue, 29 Feb 2000
Time.local(2000, 10, 31).prev_quarter # => Mon, 30 Oct 2000
Time.local(2000, 11, 31).next_quarter # => Wed, 28 Feb 2001

prev_quarter is aliased to last_quarter. beginning_of_week, end_of_week

The methods beginning_of_week and end_of_week return the dates for the beginning and end of the week, respectively. Weeks are assumed to start on Monday, but that can be changed passing an argument, setting thread local Date.beginning_of_week or config.beginning_of_week.

d =, 5, 8)     # => Sat, 08 May 2010
d.beginning_of_week          # => Mon, 03 May 2010
d.beginning_of_week(:sunday) # => Sun, 02 May 2010
d.end_of_week                # => Sun, 09 May 2010
d.end_of_week(:sunday)       # => Sat, 08 May 2010

beginning_of_week is aliased to at_beginning_of_week and end_of_week is aliased to at_end_of_week. monday, sunday

The methods monday and sunday return the dates for the previous Monday and next Sunday, respectively.

d =, 5, 8)     # => Sat, 08 May 2010
d.monday                     # => Mon, 03 May 2010
d.sunday                     # => Sun, 09 May 2010

d =, 9, 10)    # => Mon, 10 Sep 2012
d.monday                     # => Mon, 10 Sep 2012

d =, 9, 16)    # => Sun, 16 Sep 2012
d.sunday                     # => Sun, 16 Sep 2012 prev_week, next_week

The method next_week receives a symbol with a day name in English (default is the thread local Date.beginning_of_week, or config.beginning_of_week, or :monday) and it returns the date corresponding to that day.

d =, 5, 9) # => Sun, 09 May 2010
d.next_week              # => Mon, 10 May 2010
d.next_week(:saturday)   # => Sat, 15 May 2010

The method prev_week is analogous:

d.prev_week              # => Mon, 26 Apr 2010
d.prev_week(:saturday)   # => Sat, 01 May 2010
d.prev_week(:friday)     # => Fri, 30 Apr 2010

prev_week is aliased to last_week.

Both next_week and prev_week work as expected when Date.beginning_of_week or config.beginning_of_week are set. beginning_of_month, end_of_month

The methods beginning_of_month and end_of_month return the dates for the beginning and end of the month:

d =, 5, 9) # => Sun, 09 May 2010
d.beginning_of_month     # => Sat, 01 May 2010
d.end_of_month           # => Mon, 31 May 2010

beginning_of_month is aliased to at_beginning_of_month, and end_of_month is aliased to at_end_of_month. beginning_of_quarter, end_of_quarter

The methods beginning_of_quarter and end_of_quarter return the dates for the beginning and end of the quarter of the receiver's calendar year:

d =, 5, 9) # => Sun, 09 May 2010
d.beginning_of_quarter   # => Thu, 01 Apr 2010
d.end_of_quarter         # => Wed, 30 Jun 2010

beginning_of_quarter is aliased to at_beginning_of_quarter, and end_of_quarter is aliased to at_end_of_quarter. beginning_of_year, end_of_year

The methods beginning_of_year and end_of_year return the dates for the beginning and end of the year:

d =, 5, 9) # => Sun, 09 May 2010
d.beginning_of_year      # => Fri, 01 Jan 2010
d.end_of_year            # => Fri, 31 Dec 2010

beginning_of_year is aliased to at_beginning_of_year, and end_of_year is aliased to at_end_of_year.

15.1.3 Other Date Computations years_ago, years_since

The method years_ago receives a number of years and returns the same date those many years ago:

date =, 6, 7)
date.years_ago(10) # => Wed, 07 Jun 2000

years_since moves forward in time:

date =, 6, 7)
date.years_since(10) # => Sun, 07 Jun 2020

If such a day does not exist, the last day of the corresponding month is returned:, 2, 29).years_ago(3)     # => Sat, 28 Feb 2009, 2, 29).years_since(3)   # => Sat, 28 Feb 2015 months_ago, months_since

The methods months_ago and months_since work analogously for months:, 4, 30).months_ago(2)   # => Sun, 28 Feb 2010, 4, 30).months_since(2) # => Wed, 30 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:, 4, 30).months_ago(2)    # => Sun, 28 Feb 2010, 12, 31).months_since(2) # => Sun, 28 Feb 2010 weeks_ago

The method weeks_ago works analogously for weeks:, 5, 24).weeks_ago(1)    # => Mon, 17 May 2010, 5, 24).weeks_ago(2)    # => Mon, 10 May 2010 advance

The most generic way to jump to other days is advance. This method receives a hash with keys :years, :months, :weeks, :days, and returns a date advanced as much as the present keys indicate:

date =, 6, 6)
date.advance(years: 1, weeks: 2)  # => Mon, 20 Jun 2011
date.advance(months: 2, days: -2) # => Wed, 04 Aug 2010

Note in the previous example that increments may be negative.

To perform the computation the method first increments years, then months, then weeks, and finally days. This order is important towards the end of months. Say for example we are at the end of February of 2010, and we want to move one month and one day forward.

The method advance advances first one month, and then one day, the result is:, 2, 28).advance(months: 1, days: 1)
# => Sun, 29 Mar 2010

While if it did it the other way around the result would be different:, 2, 28).advance(days: 1).advance(months: 1)
# => Thu, 01 Apr 2010

15.1.4 Changing Components

The method change allows you to get a new date which is the same as the receiver except for the given year, month, or day:, 12, 23).change(year: 2011, month: 11)
# => Wed, 23 Nov 2011

This method is not tolerant to non-existing dates, if the change is invalid ArgumentError is raised:, 1, 31).change(month: 2)
# => ArgumentError: invalid date

15.1.5 Durations

Durations can be added to and subtracted from dates:

d = Date.current
# => Mon, 09 Aug 2010
d + 1.year
# => Tue, 09 Aug 2011
d - 3.hours
# => Sun, 08 Aug 2010 21:00:00 UTC +00:00

They translate to calls to since or advance. For example here we get the correct jump in the calendar reform:, 10, 4) +
# => Fri, 15 Oct 1582

15.1.6 Timestamps

The following methods return a Time object if possible, otherwise a DateTime. If set, they honor the user time zone. beginning_of_day, end_of_day

The method beginning_of_day returns a timestamp at the beginning of the day (00:00:00):

date =, 6, 7)
date.beginning_of_day # => Mon Jun 07 00:00:00 +0200 2010

The method end_of_day returns a timestamp at the end of the day (23:59:59):

date =, 6, 7)
date.end_of_day # => Mon Jun 07 23:59:59 +0200 2010

beginning_of_day is aliased to at_beginning_of_day, midnight, at_midnight. beginning_of_hour, end_of_hour

The method beginning_of_hour returns a timestamp at the beginning of the hour (hh:00:00):

date =, 6, 7, 19, 55, 25)
date.beginning_of_hour # => Mon Jun 07 19:00:00 +0200 2010

The method end_of_hour returns a timestamp at the end of the hour (hh:59:59):

date =, 6, 7, 19, 55, 25)
date.end_of_hour # => Mon Jun 07 19:59:59 +0200 2010

beginning_of_hour is aliased to at_beginning_of_hour. beginning_of_minute, end_of_minute

The method beginning_of_minute returns a timestamp at the beginning of the minute (hh:mm:00):

date =, 6, 7, 19, 55, 25)
date.beginning_of_minute # => Mon Jun 07 19:55:00 +0200 2010

The method end_of_minute returns a timestamp at the end of the minute (hh:mm:59):

date =, 6, 7, 19, 55, 25)
date.end_of_minute # => Mon Jun 07 19:55:59 +0200 2010

beginning_of_minute is aliased to at_beginning_of_minute.

beginning_of_hour, end_of_hour, beginning_of_minute and end_of_minute are implemented for Time and DateTime but not Date as it does not make sense to request the beginning or end of an hour or minute on a Date instance. ago, since

The method ago receives a number of seconds as argument and returns a timestamp those many seconds ago from midnight:

date = Date.current # => Fri, 11 Jun 2010
date.ago(1)         # => Thu, 10 Jun 2010 23:59:59 EDT -04:00

Similarly, since moves forward:

date = Date.current # => Fri, 11 Jun 2010
date.since(1)       # => Fri, 11 Jun 2010 00:00:01 EDT -04:00

15.1.7 Other Time Computations

15.2 Conversions

16 Extensions to DateTime

DateTime is not aware of DST rules and so some of these methods have edge cases when a DST change is going on. For example seconds_since_midnight might not return the real amount in such a day.

16.1 Calculations

All the following methods are defined in active_support/core_ext/date_time/calculations.rb.

The class DateTime is a subclass of Date so by loading active_support/core_ext/date/calculations.rb you inherit these methods and their aliases, except that they will always return datetimes:

beginning_of_week (at_beginning_of_week)
end_of_week (at_end_of_week)
prev_week (last_week)
beginning_of_month (at_beginning_of_month)
end_of_month (at_end_of_month)
prev_month (last_month)
beginning_of_quarter (at_beginning_of_quarter)
end_of_quarter (at_end_of_quarter)
beginning_of_year (at_beginning_of_year)
end_of_year (at_end_of_year)
prev_year (last_year)

The following methods are reimplemented so you do not need to load active_support/core_ext/date/calculations.rb for these ones:

beginning_of_day (midnight, at_midnight, at_beginning_of_day)
since (in)

On the other hand, advance and change are also defined and support more options, they are documented below.

The following methods are only implemented in active_support/core_ext/date_time/calculations.rb as they only make sense when used with a DateTime instance:

beginning_of_hour (at_beginning_of_hour)

16.1.1 Named Datetimes DateTime.current

Active Support defines DateTime.current to be like, except that it honors the user time zone, if defined. It also defines DateTime.yesterday and DateTime.tomorrow, and the instance predicates past?, and future? relative to DateTime.current.

16.1.2 Other Extensions seconds_since_midnight

The method seconds_since_midnight returns the number of seconds since midnight:

now = DateTime.current     # => Mon, 07 Jun 2010 20:26:36 +0000
now.seconds_since_midnight # => 73596 utc

The method utc gives you the same datetime in the receiver expressed in UTC.

now = DateTime.current # => Mon, 07 Jun 2010 19:27:52 -0400
now.utc                # => Mon, 07 Jun 2010 23:27:52 +0000

This method is also aliased as getutc. utc?

The predicate utc? says whether the receiver has UTC as its time zone:

now = # => Mon, 07 Jun 2010 19:30:47 -0400
now.utc?           # => false
now.utc.utc?       # => true advance

The most generic way to jump to another datetime is advance. This method receives a hash with keys :years, :months, :weeks, :days, :hours, :minutes, and :seconds, and returns a datetime advanced as much as the present keys indicate.

d = DateTime.current
# => Thu, 05 Aug 2010 11:33:31 +0000
d.advance(years: 1, months: 1, days: 1, hours: 1, minutes: 1, seconds: 1)
# => Tue, 06 Sep 2011 12:34:32 +0000

This method first computes the destination date passing :years, :months, :weeks, and :days to Date#advance documented above. After that, it adjusts the time calling since with the number of seconds to advance. This order is relevant, a different ordering would give different datetimes in some edge-cases. The example in Date#advance applies, and we can extend it to show order relevance related to the time bits.

If we first move the date bits (that have also a relative order of processing, as documented before), and then the time bits we get for example the following computation:

d =, 2, 28, 23, 59, 59)
# => Sun, 28 Feb 2010 23:59:59 +0000
d.advance(months: 1, seconds: 1)
# => Mon, 29 Mar 2010 00:00:00 +0000

but if we computed them the other way around, the result would be different:

d.advance(seconds: 1).advance(months: 1)
# => Thu, 01 Apr 2010 00:00:00 +0000

Since DateTime is not DST-aware you can end up in a non-existing point in time with no warning or error telling you so.

16.1.3 Changing Components

The method change allows you to get a new datetime which is the same as the receiver except for the given options, which may include :year, :month, :day, :hour, :min, :sec, :offset, :start:

now = DateTime.current
# => Tue, 08 Jun 2010 01:56:22 +0000
now.change(year: 2011, offset: Rational(-6, 24))
# => Wed, 08 Jun 2011 01:56:22 -0600

If hours are zeroed, then minutes and seconds are too (unless they have given values):

now.change(hour: 0)
# => Tue, 08 Jun 2010 00:00:00 +0000

Similarly, if minutes are zeroed, then seconds are too (unless it has given a value):

now.change(min: 0)
# => Tue, 08 Jun 2010 01:00:00 +0000

This method is not tolerant to non-existing dates, if the change is invalid ArgumentError is raised:

DateTime.current.change(month: 2, day: 30)
# => ArgumentError: invalid date

16.1.4 Durations

Durations can be added to and subtracted from datetimes:

now = DateTime.current
# => Mon, 09 Aug 2010 23:15:17 +0000
now + 1.year
# => Tue, 09 Aug 2011 23:15:17 +0000
now - 1.week
# => Mon, 02 Aug 2010 23:15:17 +0000

They translate to calls to since or advance. For example here we get the correct jump in the calendar reform:, 10, 4, 23) + 1.hour
# => Fri, 15 Oct 1582 00:00:00 +0000

17 Extensions to Time

17.1 Calculations

All the following methods are defined in active_support/core_ext/time/calculations.rb.

Active Support adds to Time many of the methods available for DateTime:

since (in)
beginning_of_day (midnight, at_midnight, at_beginning_of_day)
beginning_of_hour (at_beginning_of_hour)
beginning_of_week (at_beginning_of_week)
end_of_week (at_end_of_week)
prev_week (last_week)
beginning_of_month (at_beginning_of_month)
end_of_month (at_end_of_month)
prev_month (last_month)
beginning_of_quarter (at_beginning_of_quarter)
end_of_quarter (at_end_of_quarter)
beginning_of_year (at_beginning_of_year)
end_of_year (at_end_of_year)
prev_year (last_year)

They are analogous. Please refer to their documentation above and take into account the following differences:

  • change accepts an additional :usec option.
  • Time understands DST, so you get correct DST calculations as in
# => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>

# In Barcelona, 2010/03/28 02:00 +0100 becomes 2010/03/28 03:00 +0200 due to DST.
t = Time.local(2010, 3, 28, 1, 59, 59)
# => Sun Mar 28 01:59:59 +0100 2010
t.advance(seconds: 1)
# => Sun Mar 28 03:00:00 +0200 2010

  • If since or ago jump to a time that can't be expressed with Time a DateTime object is returned instead.
17.1.1 Time.current

Active Support defines Time.current to be today in the current time zone. That's like, except that it honors the user time zone, if defined. It also defines Time.yesterday and Time.tomorrow, and the instance predicates past?, today?, and future?, all of them relative to Time.current.

When making Time comparisons using methods which honor the user time zone, make sure to use Time.current and not There are cases where the user time zone might be in the future compared to the system time zone, which uses by default. This means may equal Time.yesterday.

17.1.2 all_day, all_week, all_month, all_quarter and all_year

The method all_day returns a range representing the whole day of the current time.

now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
# => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Mon, 09 Aug 2010 23:59:59 UTC +00:00

Analogously, all_week, all_month, all_quarter and all_year all serve the purpose of generating time ranges.

now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
# => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Sun, 15 Aug 2010 23:59:59 UTC +00:00
# => Sun, 16 Sep 2012 00:00:00 UTC +00:00..Sat, 22 Sep 2012 23:59:59 UTC +00:00
# => Sat, 01 Aug 2010 00:00:00 UTC +00:00..Tue, 31 Aug 2010 23:59:59 UTC +00:00
# => Thu, 01 Jul 2010 00:00:00 UTC +00:00..Thu, 30 Sep 2010 23:59:59 UTC +00:00
# => Fri, 01 Jan 2010 00:00:00 UTC +00:00..Fri, 31 Dec 2010 23:59:59 UTC +00:00

17.2 Time Constructors

Active Support defines Time.current to be if there's a user time zone defined, with fallback to

# => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>
# => Fri, 06 Aug 2010 17:11:58 CEST +02:00

Analogously to DateTime, the predicates past?, and future? are relative to Time.current.

If the time to be constructed lies beyond the range supported by Time in the runtime platform, usecs are discarded and a DateTime object is returned instead.

17.2.1 Durations

Durations can be added to and subtracted from time objects:

now = Time.current
# => Mon, 09 Aug 2010 23:20:05 UTC +00:00
now + 1.year
#  => Tue, 09 Aug 2011 23:21:11 UTC +00:00
now - 1.week
# => Mon, 02 Aug 2010 23:21:11 UTC +00:00

They translate to calls to since or advance. For example here we get the correct jump in the calendar reform:

Time.utc(1582, 10, 3) + 5.days
# => Mon Oct 18 00:00:00 UTC 1582

18 Extensions to File

18.1 atomic_write

With the class method File.atomic_write you can write to a file in a way that will prevent any reader from seeing half-written content.

The name of the file is passed as an argument, and the method yields a file handle opened for writing. Once the block is done atomic_write closes the file handle and completes its job.

For example, Action Pack uses this method to write asset cache files like all.css:

File.atomic_write(joined_asset_path) do |cache|

To accomplish this atomic_write creates a temporary file. That's the file the code in the block actually writes to. On completion, the temporary file is renamed, which is an atomic operation on POSIX systems. If the target file exists atomic_write overwrites it and keeps owners and permissions. However there are a few cases where atomic_write cannot change the file ownership or permissions, this error is caught and skipped over trusting in the user/filesystem to ensure the file is accessible to the processes that need it.

Due to the chmod operation atomic_write performs, if the target file has an ACL set on it this ACL will be recalculated/modified.

Note you can't append with atomic_write.

The auxiliary file is written in a standard directory for temporary files, but you can pass a directory of your choice as second argument.

Defined in active_support/core_ext/file/atomic.rb.

19 Extensions to Marshal

19.1 load

Active Support adds constant autoloading support to load.

For example, the file cache store deserializes this way: { |f| Marshal.load(f) }

If the cached data refers to a constant that is unknown at that point, the autoloading mechanism is triggered and if it succeeds the deserialization is retried transparently.

If the argument is an IO it needs to respond to rewind to be able to retry. Regular files respond to rewind.

Defined in active_support/core_ext/marshal.rb.

20 Extensions to Logger

20.1 around_[level]

Takes two arguments, a before_message and after_message and calls the current level method on the Logger instance, passing in the before_message, then the specified message, then the after_message:

logger ="log/development.log")
logger.around_info("before", "after") { |logger|"during") }

20.2 silence

Silences every log level lesser to the specified one for the duration of the given block. Log level orders are: debug, info, error and fatal.

logger ="log/development.log")
logger.silence(Logger::INFO) do
  logger.debug("In space, no one can hear you scream.")"Scream all you want, small mailman!")

20.3 datetime_format=

Modifies the datetime format output by the formatter class associated with this logger. If the formatter class does not have a datetime_format method then this is ignored.

class Logger::FormatWithTime < Logger::Formatter
  cattr_accessor(:datetime_format) { "%Y%m%d%H%m%S" }

  def, timestamp, progname, msg)
    "#{timestamp.strftime(datetime_format)} -- #{String === msg ? msg : msg.inspect}\n"

logger ="log/development.log")
logger.formatter = Logger::FormatWithTime"<- is the current time")

Defined in active_support/core_ext/logger.rb.

21 Extensions to NameError

Active Support adds missing_name? to NameError, which tests whether the exception was raised because of the name passed as argument.

The name may be given as a symbol or string. A symbol is tested against the bare constant name, a string is against the fully-qualified constant name.

A symbol can represent a fully-qualified constant name as in :"ActiveRecord::Base", so the behavior for symbols is defined for convenience, not because it has to be that way technically.

For example, when an action of PostsController is called Rails tries optimistically to use PostsHelper. It is OK that the helper module does not exist, so if an exception for that constant name is raised it should be silenced. But it could be the case that posts_helper.rb raises a NameError due to an actual unknown constant. That should be reraised. The method missing_name? provides a way to distinguish both cases:

def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue MissingSourceFile => e
  raise e unless e.is_missing? "#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"

Defined in active_support/core_ext/name_error.rb.

22 Extensions to LoadError

Active Support adds is_missing? to LoadError, and also assigns that class to the constant MissingSourceFile for backwards compatibility.

Given a path name is_missing? tests whether the exception was raised due to that particular file (except perhaps for the ".rb" extension).

For example, when an action of PostsController is called Rails tries to load posts_helper.rb, but that file may not exist. That's fine, the helper module is not mandatory so Rails silences a load error. But it could be the case that the helper module does exist and in turn requires another library that is missing. In that case Rails must reraise the exception. The method is_missing? provides a way to distinguish both cases:

def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue MissingSourceFile => e
  raise e unless e.is_missing? "helpers/#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"

Defined in active_support/core_ext/load_error.rb.


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