More at More Ruby on Rails

A Guide for Upgrading Ruby on Rails

This guide provides steps to be followed when you upgrade your applications to a newer version of Ruby on Rails. These steps are also available in individual release guides.


  1. General Advice
  2. Upgrading from Rails 4.1 to Rails 4.2
  3. Upgrading from Rails 4.0 to Rails 4.1
  4. Upgrading from Rails 3.2 to Rails 4.0
  5. Upgrading from Rails 3.1 to Rails 3.2
  6. Upgrading from Rails 3.0 to Rails 3.1

1 General Advice

Before attempting to upgrade an existing application, you should be sure you have a good reason to upgrade. You need to balance several factors: the need for new features, the increasing difficulty of finding support for old code, and your available time and skills, to name a few.

1.1 Test Coverage

The best way to be sure that your application still works after upgrading is to have good test coverage before you start the process. If you don't have automated tests that exercise the bulk of your application, you'll need to spend time manually exercising all the parts that have changed. In the case of a Rails upgrade, that will mean every single piece of functionality in the application. Do yourself a favor and make sure your test coverage is good before you start an upgrade.

1.2 Ruby Versions

Rails generally stays close to the latest released Ruby version when it's released:

  • Rails 3 and above require Ruby 1.8.7 or higher. Support for all of the previous Ruby versions has been dropped officially. You should upgrade as early as possible.
  • Rails 3.2.x is the last branch to support Ruby 1.8.7.
  • Rails 4 prefers Ruby 2.0 and requires 1.9.3 or newer.

Ruby 1.8.7 p248 and p249 have marshaling bugs that crash Rails. Ruby Enterprise Edition has these fixed since the release of 1.8.7-2010.02. On the 1.9 front, Ruby 1.9.1 is not usable because it outright segfaults, so if you want to use 1.9.x, jump straight to 1.9.3 for smooth sailing.

1.3 The Rake Task

Rails provides the rails:update rake task. After updating the Rails version in the Gemfile, run this rake task. This will help you with the creation of new files and changes of old files in an interactive session.

$ rake rails:update
   identical  config/boot.rb
       exist  config
    conflict  config/routes.rb
Overwrite /myapp/config/routes.rb? (enter "h" for help) [Ynaqdh]
       force  config/routes.rb
    conflict  config/application.rb
Overwrite /myapp/config/application.rb? (enter "h" for help) [Ynaqdh]
       force  config/application.rb
    conflict  config/environment.rb

Don't forget to review the difference, to see if there were any unexpected changes.

2 Upgrading from Rails 4.1 to Rails 4.2

2.1 Web Console

First, add gem 'web-console', '~> 2.0' to the :development group in your Gemfile and run bundle install (it won't have been included when you upgraded Rails). Once it's been installed, you can simply drop a reference to the console helper (i.e., <%= console %>) into any view you want to enable it for. A console will also be provided on any error page you view in your development environment.

2.2 Responders

respond_with and the class-level respond_to methods have been extracted to the responders gem. To use them, simply add gem 'responders', '~> 2.0' to your Gemfile. Calls to respond_with and respond_to (again, at the class level) will no longer work without having included the responders gem in your dependencies:

# app/controllers/users_controller.rb

class UsersController < ApplicationController
  respond_to :html, :json

  def show
    @user = User.find(params[:id])
    respond_with @user

Instance-level respond_to is unaffected and does not require the additional gem:

# app/controllers/users_controller.rb

class UsersController < ApplicationController
  def show
    @user = User.find(params[:id])
    respond_to do |format|
      format.json { render json: @user }

See #16526 for more details.

2.3 Error handling in transaction callbacks

Currently, Active Record suppresses errors raised within after_rollback or after_commit callbacks and only prints them to the logs. In the next version, these errors will no longer be suppressed. Instead, the errors will propagate normally just like in other Active Record callbacks.

When you define a after_rollback or after_commit callback, you will receive a deprecation warning about this upcoming change. When you are ready, you can opt into the new behavior and remove the deprecation warning by adding following configuration to your config/application.rb:

config.active_record.raise_in_transactional_callbacks = true

See #14488 and #16537 for more details.

2.4 Ordering of test cases

In Rails 5.0, test cases will be executed in random order by default. In anticipation of this change, Rails 4.2 introduced a new configuration option active_support.test_order for explicitly specifying the test ordering. This allows you to either lock down the current behavior by setting the option to :sorted, or opt into the future behavior by setting the option to :random.

If you do not specify a value for this option, a deprecation warning will be emitted. To avoid this, add the following line to your test environment:

# config/environments/test.rb
Rails.application.configure do
  config.active_support.test_order = :sorted # or `:random` if you prefer

2.5 Serialized attributes

When using a custom coder (e.g. serialize :metadata, JSON), assigning nil to a serialized attribute will save it to the database as NULL instead of passing the nil value through the coder (e.g. "null" when using the JSON coder).

2.6 Production log level

In Rails 5, the default log level for the production environment will be changed to :debug (from :info). To preserve the current default, add the following line to your production.rb:

# Set to `:info` to match the current default, or set to `:debug` to opt-into
# the future default.
config.log_level = :info

2.7 after_bundle in Rails templates

If you have a Rails template that adds all the files in version control, it fails to add the generated binstubs because it gets executed before Bundler:

# template.rb
generate(:scaffold, "person name:string")
route "root to: 'people#index'"

git :init
git add: "."
git commit: %Q{ -m 'Initial commit' }

You can now wrap the git calls in an after_bundle block. It will be run after the binstubs have been generated.

# template.rb
generate(:scaffold, "person name:string")
route "root to: 'people#index'"

after_bundle do
  git :init
  git add: "."
  git commit: %Q{ -m 'Initial commit' }

2.8 Rails HTML Sanitizer

There's a new choice for sanitizing HTML fragments in your applications. The venerable html-scanner approach is now officially being deprecated in favor of Rails HTML Sanitizer.

This means the methods sanitize, sanitize_css, strip_tags and strip_links are backed by a new implementation.

This new sanitizer uses Loofah internally. Loofah in turn uses Nokogiri, which wraps XML parsers written in both C and Java, so sanitization should be faster no matter which Ruby version you run.

The new version updates sanitize, so it can take a Loofah::Scrubber for powerful scrubbing. See some examples of scrubbers here.

Two new scrubbers have also been added: PermitScrubber and TargetScrubber. Read the gem's readme for more information.

The documentation for PermitScrubber and TargetScrubber explains how you can gain complete control over when and how elements should be stripped.

If your application needs to use the old sanitizer implementation, include rails-deprecated_sanitizer in your Gemfile:

gem 'rails-deprecated_sanitizer'

2.9 Rails DOM Testing

The TagAssertions module (containing methods such as assert_tag), has been deprecated in favor of the assert_select methods from the SelectorAssertions module, which has been extracted into the rails-dom-testing gem.

2.10 Masked Authenticity Tokens

In order to mitigate SSL attacks, form_authenticity_token is now masked so that it varies with each request. Thus, tokens are validated by unmasking and then decrypting. As a result, any strategies for verifying requests from non-rails forms that relied on a static session CSRF token have to take this into account.

2.11 Action Mailer

Previously, calling a mailer method on a mailer class will result in the corresponding instance method being executed directly. With the introduction of Active Job and #deliver_later, this is no longer true. In Rails 4.2, the invocation of the instance methods are deferred until either deliver_now or deliver_later is called. For example:

class Notifier < ActionMailer::Base
  def notify(user, ...)
    puts "Called"
    mail(to:, ...)

mail = Notifier.notify(user, ...) # Notifier#welcome is not yet called at this point
mail = mail.deliver_now           # Prints "Called"

This should not result in any noticible differnces for most applications. However, if you need some non-mailer methods to be exectuted synchronously, and you were previously relying on the synchronous proxying behavior, you should define them as class methods on the mailer class directly:

class Notifier < ActionMailer::Base
  def self.broadcast_notifications(users, ...)
    users.each { |user| Notifier.notify(user, ...) }

2.12 Foreign Key Support

The migration DSL has been expanded to support foreign key definitions. If you've been using the Foreigner gem, you might want to consider removing it. Note that the foreign key support of Rails is a subset of Foreigner. This means that not every Foreigner definition can be fully replaced by it's Rails migration DSL counterpart.

The migration procedure is as follows:

  1. remove gem "foreigner" from the Gemfile.
  2. run bundle install.
  3. run bin/rake db:schema:dump.
  4. make sure that db/schema.rb contains every foreign key definition with the necessary options.

3 Upgrading from Rails 4.0 to Rails 4.1

3.1 CSRF protection from remote <script> tags

Or, "whaaat my tests are failing!!!?"

Cross-site request forgery (CSRF) protection now covers GET requests with JavaScript responses, too. This prevents a third-party site from referencing your JavaScript URL and attempting to run it to extract sensitive data.

This means that your functional and integration tests that use

get :index, format: :js

will now trigger CSRF protection. Switch to

xhr :get, :index, format: :js

to explicitly test an XmlHttpRequest.

If you really mean to load JavaScript from remote <script> tags, skip CSRF protection on that action.

3.2 Spring

If you want to use Spring as your application preloader you need to:

  1. Add gem 'spring', group: :development to your Gemfile.
  2. Install spring using bundle install.
  3. Springify your binstubs with bundle exec spring binstub --all.

User defined rake tasks will run in the development environment by default. If you want them to run in other environments consult the Spring README.

3.3 config/secrets.yml

If you want to use the new secrets.yml convention to store your application's secrets, you need to:

  1. Create a secrets.yml file in your config folder with the following content:

      secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>
  2. Use your existing secret_key_base from the secret_token.rb initializer to set the SECRET_KEY_BASE environment variable for whichever users running the Rails application in production mode. Alternatively, you can simply copy the existing secret_key_base from the secret_token.rb initializer to secrets.yml under the production section, replacing '<%= ENV["SECRET_KEY_BASE"] %>'.

  3. Remove the secret_token.rb initializer.

  4. Use rake secret to generate new keys for the development and test sections.

  5. Restart your server.

3.4 Changes to test helper

If your test helper contains a call to ActiveRecord::Migration.check_pending! this can be removed. The check is now done automatically when you require 'rails/test_help', although leaving this line in your helper is not harmful in any way.

3.5 Cookies serializer

Applications created before Rails 4.1 uses Marshal to serialize cookie values into the signed and encrypted cookie jars. If you want to use the new JSON-based format in your application, you can add an initializer file with the following content:

Rails.application.config.action_dispatch.cookies_serializer = :hybrid

This would transparently migrate your existing Marshal-serialized cookies into the new JSON-based format.

When using the :json or :hybrid serializer, you should beware that not all Ruby objects can be serialized as JSON. For example, Date and Time objects will be serialized as strings, and Hashes will have their keys stringified.

class CookiesController < ApplicationController
  def set_cookie
    cookies.encrypted[:expiration_date] = Date.tomorrow # => Thu, 20 Mar 2014
    redirect_to action: 'read_cookie'

  def read_cookie
    cookies.encrypted[:expiration_date] # => "2014-03-20"

It's advisable that you only store simple data (strings and numbers) in cookies. If you have to store complex objects, you would need to handle the conversion manually when reading the values on subsequent requests.

If you use the cookie session store, this would apply to the session and flash hash as well.

3.6 Flash structure changes

Flash message keys are normalized to strings. They can still be accessed using either symbols or strings. Looping through the flash will always yield string keys:

flash["string"] = "a string"
flash[:symbol] = "a symbol"

# Rails < 4.1
flash.keys # => ["string", :symbol]

# Rails >= 4.1
flash.keys # => ["string", "symbol"]

Make sure you are comparing Flash message keys against strings.

3.7 Changes in JSON handling

There are a few major changes related to JSON handling in Rails 4.1.

3.7.1 MultiJSON removal

MultiJSON has reached its end-of-life and has been removed from Rails.

If your application currently depend on MultiJSON directly, you have a few options:

  1. Add 'multi_json' to your Gemfile. Note that this might cease to work in the future

  2. Migrate away from MultiJSON by using obj.to_json, and JSON.parse(str) instead.

Do not simply replace MultiJson.dump and MultiJson.load with JSON.dump and JSON.load. These JSON gem APIs are meant for serializing and deserializing arbitrary Ruby objects and are generally unsafe.

3.7.2 JSON gem compatibility

Historically, Rails had some compatibility issues with the JSON gem. Using JSON.generate and JSON.dump inside a Rails application could produce unexpected errors.

Rails 4.1 fixed these issues by isolating its own encoder from the JSON gem. The JSON gem APIs will function as normal, but they will not have access to any Rails-specific features. For example:

class FooBar
  def as_json(options = nil)
    { foo: 'bar' }

>> # => "{\"foo\":\"bar\"}"
>> JSON.generate(, quirks_mode: true) # => "\"#<FooBar:0x007fa80a481610>\""

3.7.3 New JSON encoder

The JSON encoder in Rails 4.1 has been rewritten to take advantage of the JSON gem. For most applications, this should be a transparent change. However, as part of the rewrite, the following features have been removed from the encoder:

  1. Circular data structure detection
  2. Support for the encode_json hook
  3. Option to encode BigDecimal objects as numbers instead of strings

If your application depends on one of these features, you can get them back by adding the activesupport-json_encoder gem to your Gemfile.

3.7.4 JSON representation of Time objects

#as_json for objects with time component (Time, DateTime, ActiveSupport::TimeWithZone) now returns millisecond precision by default. If you need to keep old behavior with no millisecond precision, set the following in an initializer:

ActiveSupport::JSON::Encoding.time_precision = 0

3.8 Usage of return within inline callback blocks

Previously, Rails allowed inline callback blocks to use return this way:

class ReadOnlyModel < ActiveRecord::Base
  before_save { return false } # BAD

This behavior was never intentionally supported. Due to a change in the internals of ActiveSupport::Callbacks, this is no longer allowed in Rails 4.1. Using a return statement in an inline callback block causes a LocalJumpError to be raised when the callback is executed.

Inline callback blocks using return can be refactored to evaluate to the returned value:

class ReadOnlyModel < ActiveRecord::Base
  before_save { false } # GOOD

Alternatively, if return is preferred it is recommended to explicitly define a method:

class ReadOnlyModel < ActiveRecord::Base
  before_save :before_save_callback # GOOD

    def before_save_callback
      return false

This change applies to most places in Rails where callbacks are used, including Active Record and Active Model callbacks, as well as filters in Action Controller (e.g. before_action).

See this pull request for more details.

3.9 Methods defined in Active Record fixtures

Rails 4.1 evaluates each fixture's ERB in a separate context, so helper methods defined in a fixture will not be available in other fixtures.

Helper methods that are used in multiple fixtures should be defined on modules included in the newly introduced ActiveRecord::FixtureSet.context_class, in test_helper.rb.

module FixtureFileHelpers
  def file_sha(path)
    Digest::SHA2.hexdigest('test/fixtures', path)))
ActiveRecord::FixtureSet.context_class.send :include, FixtureFileHelpers

3.10 I18n enforcing available locales

Rails 4.1 now defaults the I18n option enforce_available_locales to true. This means that it will make sure that all locales passed to it must be declared in the available_locales list.

To disable it (and allow I18n to accept any locale option) add the following configuration to your application:

config.i18n.enforce_available_locales = false

Note that this option was added as a security measure, to ensure user input cannot be used as locale information unless it is previously known. Therefore, it's recommended not to disable this option unless you have a strong reason for doing so.

3.11 Mutator methods called on Relation

Relation no longer has mutator methods like #map! and #delete_if. Convert to an Array by calling #to_a before using these methods.

It intends to prevent odd bugs and confusion in code that call mutator methods directly on the Relation.

# Instead of this
Author.where(name: 'Hank Moody').compact!

# Now you have to do this
authors = Author.where(name: 'Hank Moody').to_a

3.12 Changes on Default Scopes

Default scopes are no longer overridden by chained conditions.

In previous versions when you defined a default_scope in a model it was overridden by chained conditions in the same field. Now it is merged like any other scope.


class User < ActiveRecord::Base
  default_scope { where state: 'pending' }
  scope :active, -> { where state: 'active' }
  scope :inactive, -> { where state: 'inactive' }

# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending'
# SELECT "users".* FROM "users" WHERE "users"."state" = 'active'

User.where(state: 'inactive')
# SELECT "users".* FROM "users" WHERE "users"."state" = 'inactive'


class User < ActiveRecord::Base
  default_scope { where state: 'pending' }
  scope :active, -> { where state: 'active' }
  scope :inactive, -> { where state: 'inactive' }

# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending'
# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending' AND "users"."state" = 'active'

User.where(state: 'inactive')
# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending' AND "users"."state" = 'inactive'

To get the previous behavior it is needed to explicitly remove the default_scope condition using unscoped, unscope, rewhere or except.

class User < ActiveRecord::Base
  default_scope { where state: 'pending' }
  scope :active, -> { unscope(where: :state).where(state: 'active') }
  scope :inactive, -> { rewhere state: 'inactive' }

# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending'
# SELECT "users".* FROM "users" WHERE "users"."state" = 'active'

# SELECT "users".* FROM "users" WHERE "users"."state" = 'inactive'

3.13 Rendering content from string

Rails 4.1 introduces :plain, :html, and :body options to render. Those options are now the preferred way to render string-based content, as it allows you to specify which content type you want the response sent as.

  • render :plain will set the content type to text/plain
  • render :html will set the content type to text/html
  • render :body will not set the content type header.

From the security standpoint, if you don't expect to have any markup in your response body, you should be using render :plain as most browsers will escape unsafe content in the response for you.

We will be deprecating the use of render :text in a future version. So please start using the more precise :plain, :html, and :body options instead. Using render :text may pose a security risk, as the content is sent as text/html.

3.14 PostgreSQL json and hstore datatypes

Rails 4.1 will map json and hstore columns to a string-keyed Ruby Hash. In earlier versions, a HashWithIndifferentAccess was used. This means that symbol access is no longer supported. This is also the case for store_accessors based on top of json or hstore columns. Make sure to use string keys consistently.

3.15 Explicit block use for ActiveSupport::Callbacks

Rails 4.1 now expects an explicit block to be passed when calling ActiveSupport::Callbacks.set_callback. This change stems from ActiveSupport::Callbacks being largely rewritten for the 4.1 release.

# Previously in Rails 4.0
set_callback :save, :around, ->(r, &block) { stuff; result =; stuff }

# Now in Rails 4.1
set_callback :save, :around, ->(r, block) { stuff; result =; stuff }

4 Upgrading from Rails 3.2 to Rails 4.0

If your application is currently on any version of Rails older than 3.2.x, you should upgrade to Rails 3.2 before attempting one to Rails 4.0.

The following changes are meant for upgrading your application to Rails 4.0.


Rails 4 now uses PATCH as the primary HTTP verb for updates when a RESTful resource is declared in config/routes.rb. The update action is still used, and PUT requests will continue to be routed to the update action as well. So, if you're using only the standard RESTful routes, no changes need to be made:

resources :users

<%= form_for @user do |f| %>

class UsersController < ApplicationController
  def update
    # No change needed; PATCH will be preferred, and PUT will still work.

However, you will need to make a change if you are using form_for to update a resource in conjunction with a custom route using the PUT HTTP method:

resources :users, do
  put :update_name, on: :member

<%= form_for [ :update_name, @user ] do |f| %>

class UsersController < ApplicationController
  def update_name
    # Change needed; form_for will try to use a non-existent PATCH route.

If the action is not being used in a public API and you are free to change the HTTP method, you can update your route to use patch instead of put:

PUT requests to /users/:id in Rails 4 get routed to update as they are today. So, if you have an API that gets real PUT requests it is going to work. The router also routes PATCH requests to /users/:id to the update action.

resources :users do
  patch :update_name, on: :member

If the action is being used in a public API and you can't change to HTTP method being used, you can update your form to use the PUT method instead:

<%= form_for [ :update_name, @user ], method: :put do |f| %>

For more on PATCH and why this change was made, see this post on the Rails blog.

4.1.1 A note about media types

The errata for the PATCH verb specifies that a 'diff' media type should be used with PATCH. One such format is JSON Patch. While Rails does not support JSON Patch natively, it's easy enough to add support:

# in your controller
def update
  respond_to do |format|
    format.json do
      # perform a partial update
      @article.update params[:article]

    format.json_patch do
      # perform sophisticated change

# In config/initializers/json_patch.rb:
Mime::Type.register 'application/json-patch+json', :json_patch

As JSON Patch was only recently made into an RFC, there aren't a lot of great Ruby libraries yet. Aaron Patterson's hana is one such gem, but doesn't have full support for the last few changes in the specification.

4.2 Gemfile

Rails 4.0 removed the assets group from Gemfile. You'd need to remove that line from your Gemfile when upgrading. You should also update your application file (in config/application.rb):

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.

4.3 vendor/plugins

Rails 4.0 no longer supports loading plugins from vendor/plugins. You must replace any plugins by extracting them to gems and adding them to your Gemfile. If you choose not to make them gems, you can move them into, say, lib/my_plugin/* and add an appropriate initializer in config/initializers/my_plugin.rb.

4.4 Active Record

  • Rails 4.0 has removed the identity map from Active Record, due to some inconsistencies with associations. If you have manually enabled it in your application, you will have to remove the following config that has no effect anymore: config.active_record.identity_map.

  • The delete method in collection associations can now receive Integer or String arguments as record ids, besides records, pretty much like the destroy method does. Previously it raised ActiveRecord::AssociationTypeMismatch for such arguments. From Rails 4.0 on delete automatically tries to find the records matching the given ids before deleting them.

  • In Rails 4.0 when a column or a table is renamed the related indexes are also renamed. If you have migrations which rename the indexes, they are no longer needed.

  • Rails 4.0 has changed serialized_attributes and attr_readonly to class methods only. You shouldn't use instance methods since it's now deprecated. You should change them to use class methods, e.g. self.serialized_attributes to self.class.serialized_attributes.

  • When using the default coder, assigning nil to a serialized attribute will save it to the database as NULL instead of passing the nil value through YAML ("--- \n...\n").

  • Rails 4.0 has removed attr_accessible and attr_protected feature in favor of Strong Parameters. You can use the Protected Attributes gem for a smooth upgrade path.

  • If you are not using Protected Attributes, you can remove any options related to this gem such as whitelist_attributes or mass_assignment_sanitizer options.

  • Rails 4.0 requires that scopes use a callable object such as a Proc or lambda:

  scope :active, where(active: true)

  # becomes
  scope :active, -> { where active: true }

  • Rails 4.0 has deprecated ActiveRecord::Fixtures in favor of ActiveRecord::FixtureSet.

  • Rails 4.0 has deprecated ActiveRecord::TestCase in favor of ActiveSupport::TestCase.

  • Rails 4.0 has deprecated the old-style hash based finder API. This means that methods which previously accepted "finder options" no longer do. For example, Book.find(:all, conditions: { name: '1984' }) has been deprecated in favor of Book.where(name: '1984')

  • All dynamic methods except for find_by_... and find_by_...! are deprecated. Here's how you can handle the changes:

    • find_all_by_... becomes where(...).
    • find_last_by_... becomes where(...).last.
    • scoped_by_... becomes where(...).
    • find_or_initialize_by_... becomes find_or_initialize_by(...).
    • find_or_create_by_... becomes find_or_create_by(...).
  • Note that where(...) returns a relation, not an array like the old finders. If you require an Array, use where(...).to_a.

  • These equivalent methods may not execute the same SQL as the previous implementation.

  • To re-enable the old finders, you can use the activerecord-deprecated_finders gem.

  • Rails 4.0 has changed to default join table for has_and_belongs_to_many relations to strip the common prefix off the second table name. Any existing has_and_belongs_to_many relationship between models with a common prefix must be specified with the join_table option. For example:

CatalogCategory < ActiveRecord::Base
  has_and_belongs_to_many :catalog_products, join_table: 'catalog_categories_catalog_products'

CatalogProduct < ActiveRecord::Base
  has_and_belongs_to_many :catalog_categories, join_table: 'catalog_categories_catalog_products'

  • Note that the the prefix takes scopes into account as well, so relations between Catalog::Category and Catalog::Product or Catalog::Category and CatalogProduct need to be updated similarly.

4.5 Active Resource

Rails 4.0 extracted Active Resource to its own gem. If you still need the feature you can add the Active Resource gem in your Gemfile.

4.6 Active Model

  • Rails 4.0 has changed how errors attach with the ActiveModel::Validations::ConfirmationValidator. Now when confirmation validations fail, the error will be attached to :#{attribute}_confirmation instead of attribute.

  • Rails 4.0 has changed ActiveModel::Serializers::JSON.include_root_in_json default value to false. Now, Active Model Serializers and Active Record objects have the same default behavior. This means that you can comment or remove the following option in the config/initializers/wrap_parameters.rb file:

# Disable root element in JSON by default.
# ActiveSupport.on_load(:active_record) do
#   self.include_root_in_json = false
# end

4.7 Action Pack

  • Rails 4.0 introduces ActiveSupport::KeyGenerator and uses this as a base from which to generate and verify signed cookies (among other things). Existing signed cookies generated with Rails 3.x will be transparently upgraded if you leave your existing secret_token in place and add the new secret_key_base.
  # config/initializers/secret_token.rb
  Myapp::Application.config.secret_token = 'existing secret token'
  Myapp::Application.config.secret_key_base = 'new secret key base'

Please note that you should wait to set secret_key_base until you have 100% of your userbase on Rails 4.x and are reasonably sure you will not need to rollback to Rails 3.x. This is because cookies signed based on the new secret_key_base in Rails 4.x are not backwards compatible with Rails 3.x. You are free to leave your existing secret_token in place, not set the new secret_key_base, and ignore the deprecation warnings until you are reasonably sure that your upgrade is otherwise complete.

If you are relying on the ability for external applications or Javascript to be able to read your Rails app's signed session cookies (or signed cookies in general) you should not set secret_key_base until you have decoupled these concerns.

  • Rails 4.0 encrypts the contents of cookie-based sessions if secret_key_base has been set. Rails 3.x signed, but did not encrypt, the contents of cookie-based session. Signed cookies are "secure" in that they are verified to have been generated by your app and are tamper-proof. However, the contents can be viewed by end users, and encrypting the contents eliminates this caveat/concern without a significant performance penalty.

Please read Pull Request #9978 for details on the move to encrypted session cookies.

  • Rails 4.0 removed the ActionController::Base.asset_path option. Use the assets pipeline feature.

  • Rails 4.0 has deprecated ActionController::Base.page_cache_extension option. Use ActionController::Base.default_static_extension instead.

  • Rails 4.0 has removed Action and Page caching from Action Pack. You will need to add the actionpack-action_caching gem in order to use caches_action and the actionpack-page_caching to use caches_pages in your controllers.

  • Rails 4.0 has removed the XML parameters parser. You will need to add the actionpack-xml_parser gem if you require this feature.

  • Rails 4.0 changes the default layout lookup set using symbols or procs that return nil. To get the "no layout" behavior, return false instead of nil.

  • Rails 4.0 changes the default memcached client from memcache-client to dalli. To upgrade, simply add gem 'dalli' to your Gemfile.

  • Rails 4.0 deprecates the dom_id and dom_class methods in controllers (they are fine in views). You will need to include the ActionView::RecordIdentifier module in controllers requiring this feature.

  • Rails 4.0 deprecates the :confirm option for the link_to helper. You should instead rely on a data attribute (e.g. data: { confirm: 'Are you sure?' }). This deprecation also concerns the helpers based on this one (such as link_to_if or link_to_unless).

  • Rails 4.0 changed how assert_generates, assert_recognizes, and assert_routing work. Now all these assertions raise Assertion instead of ActionController::RoutingError.

  • Rails 4.0 raises an ArgumentError if clashing named routes are defined. This can be triggered by explicitly defined named routes or by the resources method. Here are two examples that clash with routes named example_path:

  get 'one' => 'test#example', as: :example
  get 'two' => 'test#example', as: :example

  resources :examples
  get 'clashing/:id' => 'test#example', as: :example

In the first case, you can simply avoid using the same name for multiple routes. In the second, you can use the only or except options provided by the resources method to restrict the routes created as detailed in the Routing Guide.

  • Rails 4.0 also changed the way unicode character routes are drawn. Now you can draw unicode character routes directly. If you already draw such routes, you must change them, for example:
get Rack::Utils.escape('こんにちは'), controller: 'welcome', action: 'index'


get 'こんにちは', controller: 'welcome', action: 'index'

  • Rails 4.0 requires that routes using match must specify the request method. For example:
  # Rails 3.x
  match '/' => 'root#index'

  # becomes
  match '/' => 'root#index', via: :get

  # or
  get '/' => 'root#index'

Remember you must also remove any references to the middleware from your application code, for example:

# Raise exception
config.middleware.insert_before(Rack::Lock, ActionDispatch::BestStandardsSupport)

Also check your environment settings for config.action_dispatch.best_standards_support and remove it if present.

  • In Rails 4.0, precompiling assets no longer automatically copies non-JS/CSS assets from vendor/assets and lib/assets. Rails application and engine developers should put these assets in app/assets or configure config.assets.precompile.

  • In Rails 4.0, ActionController::UnknownFormat is raised when the action doesn't handle the request format. By default, the exception is handled by responding with 406 Not Acceptable, but you can override that now. In Rails 3, 406 Not Acceptable was always returned. No overrides.

  • In Rails 4.0, a generic ActionDispatch::ParamsParser::ParseError exception is raised when ParamsParser fails to parse request params. You will want to rescue this exception instead of the low-level MultiJson::DecodeError, for example.

  • In Rails 4.0, SCRIPT_NAME is properly nested when engines are mounted on an app that's served from a URL prefix. You no longer have to set default_url_options[:script_name] to work around overwritten URL prefixes.

  • Rails 4.0 deprecated ActionController::Integration in favor of ActionDispatch::Integration.

  • Rails 4.0 deprecated ActionController::IntegrationTest in favor of ActionDispatch::IntegrationTest.

  • Rails 4.0 deprecated ActionController::PerformanceTest in favor of ActionDispatch::PerformanceTest.

  • Rails 4.0 deprecated ActionController::AbstractRequest in favor of ActionDispatch::Request.

  • Rails 4.0 deprecated ActionController::Request in favor of ActionDispatch::Request.

  • Rails 4.0 deprecated ActionController::AbstractResponse in favor of ActionDispatch::Response.

  • Rails 4.0 deprecated ActionController::Response in favor of ActionDispatch::Response.

  • Rails 4.0 deprecated ActionController::Routing in favor of ActionDispatch::Routing.

4.8 Active Support

Rails 4.0 removes the j alias for ERB::Util#json_escape since j is already used for ActionView::Helpers::JavaScriptHelper#escape_javascript.

4.9 Helpers Loading Order

The order in which helpers from more than one directory are loaded has changed in Rails 4.0. Previously, they were gathered and then sorted alphabetically. After upgrading to Rails 4.0, helpers will preserve the order of loaded directories and will be sorted alphabetically only within each directory. Unless you explicitly use the helpers_path parameter, this change will only impact the way of loading helpers from engines. If you rely on the ordering, you should check if correct methods are available after upgrade. If you would like to change the order in which engines are loaded, you can use config.railties_order= method.

4.10 Active Record Observer and Action Controller Sweeper

ActiveRecord::Observer and ActionController::Caching::Sweeper have been extracted to the rails-observers gem. You will need to add the rails-observers gem if you require these features.

4.11 sprockets-rails

  • assets:precompile:primary and assets:precompile:all have been removed. Use assets:precompile instead.
  • The config.assets.compress option should be changed to config.assets.js_compressor like so for instance:
config.assets.js_compressor = :uglifier

4.12 sass-rails

  • asset-url with two arguments is deprecated. For example: asset-url("rails.png", image) becomes asset-url("rails.png").

5 Upgrading from Rails 3.1 to Rails 3.2

If your application is currently on any version of Rails older than 3.1.x, you should upgrade to Rails 3.1 before attempting an update to Rails 3.2.

The following changes are meant for upgrading your application to the latest 3.2.x version of Rails.

5.1 Gemfile

Make the following changes to your Gemfile.

gem 'rails', '3.2.18'

group :assets do
  gem 'sass-rails',   '~> 3.2.6'
  gem 'coffee-rails', '~> 3.2.2'
  gem 'uglifier',     '>= 1.0.3'

5.2 config/environments/development.rb

There are a couple of new configuration settings that you should add to your development environment:

# Raise exception on mass assignment protection for Active Record models
config.active_record.mass_assignment_sanitizer = :strict

# Log the query plan for queries taking more than this (works
# with SQLite, MySQL, and PostgreSQL)
config.active_record.auto_explain_threshold_in_seconds = 0.5

5.3 config/environments/test.rb

The mass_assignment_sanitizer configuration setting should also be be added to config/environments/test.rb:

# Raise exception on mass assignment protection for Active Record models
config.active_record.mass_assignment_sanitizer = :strict

5.4 vendor/plugins

Rails 3.2 deprecates vendor/plugins and Rails 4.0 will remove them completely. While it's not strictly necessary as part of a Rails 3.2 upgrade, you can start replacing any plugins by extracting them to gems and adding them to your Gemfile. If you choose not to make them gems, you can move them into, say, lib/my_plugin/* and add an appropriate initializer in config/initializers/my_plugin.rb.

5.5 Active Record

Option :dependent => :restrict has been removed from belongs_to. If you want to prevent deleting the object if there are any associated objects, you can set :dependent => :destroy and return false after checking for existence of association from any of the associated object's destroy callbacks.

6 Upgrading from Rails 3.0 to Rails 3.1

If your application is currently on any version of Rails older than 3.0.x, you should upgrade to Rails 3.0 before attempting an update to Rails 3.1.

The following changes are meant for upgrading your application to Rails 3.1.12, the last 3.1.x version of Rails.

6.1 Gemfile

Make the following changes to your Gemfile.

gem 'rails', '3.1.12'
gem 'mysql2'

# Needed for the new asset pipeline
group :assets do
  gem 'sass-rails',   '~> 3.1.7'
  gem 'coffee-rails', '~> 3.1.1'
  gem 'uglifier',     '>= 1.0.3'

# jQuery is the default JavaScript library in Rails 3.1
gem 'jquery-rails'

6.2 config/application.rb

The asset pipeline requires the following additions:

config.assets.enabled = true
config.assets.version = '1.0'

If your application is using an "/assets" route for a resource you may want change the prefix used for assets to avoid conflicts:

# Defaults to '/assets'
config.assets.prefix = '/asset-files'

6.3 config/environments/development.rb

Remove the RJS setting config.action_view.debug_rjs = true.

Add these settings if you enable the asset pipeline:

# Do not compress assets
config.assets.compress = false

# Expands the lines which load the assets
config.assets.debug = true

6.4 config/environments/production.rb

Again, most of the changes below are for the asset pipeline. You can read more about these in the Asset Pipeline guide.

# Compress JavaScripts and CSS
config.assets.compress = true

# Don't fallback to assets pipeline if a precompiled asset is missed
config.assets.compile = false

# Generate digests for assets URLs
config.assets.digest = true

# Defaults to Rails.root.join("public/assets")
# config.assets.manifest = YOUR_PATH

# Precompile additional assets (application.js, application.css, and all non-JS/CSS are already added)
# config.assets.precompile += %w( search.js )

# Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
# config.force_ssl = true

6.5 config/environments/test.rb

You can help test performance with these additions to your test environment:

# Configure static asset server for tests with Cache-Control for performance
config.serve_static_files = true
config.static_cache_control = 'public, max-age=3600'

6.6 config/initializers/wrap_parameters.rb

Add this file with the following contents, if you wish to wrap parameters into a nested hash. This is on by default in new applications.

# Be sure to restart your server when you modify this file.
# This file contains settings for ActionController::ParamsWrapper which
# is enabled by default.

# Enable parameter wrapping for JSON. You can disable this by setting :format to an empty array.
ActiveSupport.on_load(:action_controller) do
  wrap_parameters format: [:json]

# Disable root element in JSON by default.
ActiveSupport.on_load(:active_record) do
  self.include_root_in_json = false

6.7 config/initializers/session_store.rb

You need to change your session key to something new, or remove all sessions:

# in config/initializers/session_store.rb
AppName::Application.config.session_store :cookie_store, key: 'SOMETHINGNEW'


$ bin/rake db:sessions:clear

6.8 Remove :cache and :concat options in asset helpers references in views

  • With the Asset Pipeline the :cache and :concat options aren't used anymore, delete these options from your views.


You're encouraged to help improve the quality of this guide.

Please contribute if you see any typos or factual errors. To get started, you can read our documentation contributions section.

You may also find incomplete content, or stuff that is not up to date. Please do add any missing documentation for master. Make sure to check Edge Guides first to verify if the issues are already fixed or not on the master branch. Check the Ruby on Rails Guides Guidelines for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.

And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome in the rubyonrails-docs mailing list.