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Ruby on Rails 4.2 Release Notes

Highlights in Rails 4.2:

These release notes cover only the major changes. To learn about other features, bug fixes, and changes, please refer to the changelogs or check out the list of commits in the main Rails repository on GitHub.

1 Upgrading to Rails 4.2

If you're upgrading an existing application, it's a great idea to have good test coverage before going in. You should also first upgrade to Rails 4.1 in case you haven't and make sure your application still runs as expected before attempting to upgrade to Rails 4.2. A list of things to watch out for when upgrading is available in the guide Upgrading Ruby on Rails.

2 Major Features

2.1 Active Job

Active Job is a new framework in Rails 4.2. It is a common interface on top of queuing systems like Resque, Delayed Job, Sidekiq, and more.

Jobs written with the Active Job API run on any of the supported queues thanks to their respective adapters. Active Job comes pre-configured with an inline runner that executes jobs right away.

Jobs often need to take Active Record objects as arguments. Active Job passes object references as URIs (uniform resource identifiers) instead of marshalling the object itself. The new Global ID library builds URIs and looks up the objects they reference. Passing Active Record objects as job arguments just works by using Global ID internally.

For example, if trashable is an Active Record object, then this job runs just fine with no serialization involved:

class TrashableCleanupJob < ActiveJob::Base
  def perform(trashable, depth)

See the Active Job Basics guide for more information.

2.2 Asynchronous Mails

Building on top of Active Job, Action Mailer now comes with a deliver_later method that sends emails via the queue, so it doesn't block the controller or model if the queue is asynchronous (the default inline queue blocks).

Sending emails right away is still possible with deliver_now.

2.3 Adequate Record

Adequate Record is a set of performance improvements in Active Record that makes common find and find_by calls and some association queries up to 2x faster.

It works by caching common SQL queries as prepared statements and reusing them on similar calls, skipping most of the query-generation work on subsequent calls. For more details, please refer to Aaron Patterson's blog post.

Active Record will automatically take advantage of this feature on supported operations without any user involvement or code changes. Here are some examples of supported operations:

Post.find(1)  # First call generates and cache the prepared statement
Post.find(2)  # Subsequent calls reuse the cached prepared statement

Post.find_by_title('first post')
Post.find_by_title('second post')

Post.find_by(title: 'first post')
Post.find_by(title: 'second post')


It's important to highlight that, as the examples above suggest, the prepared statements do not cache the values passed in the method calls; rather, they have placeholders for them.

Caching is not used in the following scenarios:

  • The model has a default scope
  • The model uses single table inheritance
  • find with a list of ids, e.g.:

    # not cached
    Post.find(1, 2, 3)
  • find_by with SQL fragments:

    Post.find_by('published_at < ?', 2.weeks.ago)

2.4 Web Console

New applications generated with Rails 4.2 now come with the Web Console gem by default. Web Console adds an interactive Ruby console on every error page and provides a console view and controller helpers.

The interactive console on error pages lets you execute code in the context of the place where the exception originated. The console helper, if called anywhere in a view or controller, launches an interactive console with the final context, once rendering has completed.

2.5 Foreign Key Support

The migration DSL now supports adding and removing foreign keys. They are dumped to schema.rb as well. At this time, only the mysql, mysql2 and postgresql adapters support foreign keys.

# add a foreign key to `articles.author_id` referencing `authors.id`
add_foreign_key :articles, :authors

# add a foreign key to `articles.author_id` referencing `users.lng_id`
add_foreign_key :articles, :users, column: :author_id, primary_key: "lng_id"

# remove the foreign key on `accounts.branch_id`
remove_foreign_key :accounts, :branches

# remove the foreign key on `accounts.owner_id`
remove_foreign_key :accounts, column: :owner_id

See the API documentation on add_foreign_key and remove_foreign_key for a full description.

3 Incompatibilities

Previously deprecated functionality has been removed. Please refer to the individual components for new deprecations in this release.

The following changes may require immediate action upon upgrade.

3.1 render with a String Argument

Previously, calling render "foo/bar" in a controller action was equivalent to render file: "foo/bar". In Rails 4.2, this has been changed to mean render template: "foo/bar" instead. If you need to render a file, please change your code to use the explicit form (render file: "foo/bar") instead.

3.2 respond_with / Class-Level respond_to

respond_with and the corresponding class-level respond_to have been moved to the responders gem. Add gem 'responders', '~> 2.0' to your Gemfile to use it:

# 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:

# app/controllers/users_controller.rb

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

3.3 Default Host for rails server

Due to a change in Rack, rails server now listens on localhost instead of by default. This should have minimal impact on the standard development workflow as both and http://localhost:3000 will continue to work as before on your own machine.

However, with this change you will no longer be able to access the Rails server from a different machine, for example if your development environment is in a virtual machine and you would like to access it from the host machine. In such cases, please start the server with rails server -b to restore the old behavior.

If you do this, be sure to configure your firewall properly such that only trusted machines on your network can access your development server.

3.4 Changed status option symbols for render

Due to a change in Rack, the symbols that the render method accepts for the :status option have changed:

  • 306: :reserved has been removed.
  • 413: :request_entity_too_large has been renamed to :payload_too_large.
  • 414: :request_uri_too_long has been renamed to :uri_too_long.
  • 416: :requested_range_not_satisfiable has been renamed to :range_not_satisfiable.

Keep in mind that if calling render with an unknown symbol, the response status will default to 500.

3.5 HTML Sanitizer

The HTML sanitizer has been replaced with a new, more robust, implementation built upon Loofah and Nokogiri. The new sanitizer is more secure and its sanitization is more powerful and flexible.

Due to the new algorithm, the sanitized output may be different for certain pathological inputs.

If you have a particular need for the exact output of the old sanitizer, you can add the rails-deprecated_sanitizer gem to the Gemfile, to have the old behavior. The gem does not issue deprecation warnings because it is opt-in.

rails-deprecated_sanitizer will be supported for Rails 4.2 only; it will not be maintained for Rails 5.0.

See this blog post for more details on the changes in the new sanitizer.

3.6 assert_select

assert_select is now based on Nokogiri. As a result, some previously-valid selectors are now unsupported. If your application is using any of these spellings, you will need to update them:

  • Values in attribute selectors may need to be quoted if they contain non-alphanumeric characters.

    # before
    # now
  • DOMs built from HTML source containing invalid HTML with improperly nested elements may differ.

    For example:

    # content: <div><i><p></i></div>
    # before:
    assert_select('div > i')  # => true
    assert_select('div > p')  # => false
    assert_select('i > p')    # => true
    # now:
    assert_select('div > i')  # => true
    assert_select('div > p')  # => true
    assert_select('i > p')    # => false
  • If the data selected contains entities, the value selected for comparison used to be raw (e.g. AT&amp;T), and now is evaluated (e.g. AT&T).

    # content: <p>AT&amp;T</p>
    # before:
    assert_select('p', 'AT&amp;T')  # => true
    assert_select('p', 'AT&T')      # => false
    # now:
    assert_select('p', 'AT&T')      # => true
    assert_select('p', 'AT&amp;T')  # => false

Furthermore substitutions have changed syntax.

Now you have to use a :match CSS-like selector:

assert_select ":match('id', ?)", 'comment_1'

Additionally Regexp substitutions look different when the assertion fails. Notice how /hello/ here:

assert_select(":match('id', ?)", /hello/)

becomes "(?-mix:hello)":

Expected at least 1 element matching "div:match('id', "(?-mix:hello)")", found 0..
Expected 0 to be >= 1.

See the Rails Dom Testing documentation for more on assert_select.

4 Railties

Please refer to the Changelog for detailed changes.

4.1 Removals

  • The --skip-action-view option has been removed from the app generator. (Pull Request)

  • The rails application command has been removed without replacement. (Pull Request)

4.2 Deprecations

  • Deprecated missing config.log_level for production environments. (Pull Request)

  • Deprecated rake test:all in favor of rake test as it now run all tests in the test folder. (Pull Request)

  • Deprecated rake test:all:db in favor of rake test:db. (Pull Request)

  • Deprecated Rails::Rack::LogTailer without replacement. (Commit)

4.3 Notable changes

  • Introduced web-console in the default application Gemfile. (Pull Request)

  • Added a required option to the model generator for associations. (Pull Request)

  • Introduced the x namespace for defining custom configuration options:

    # config/environments/production.rb
    config.x.payment_processing.schedule = :daily
    config.x.payment_processing.retries  = 3
    config.x.super_debugger              = true

    These options are then available through the configuration object:

    Rails.configuration.x.payment_processing.schedule # => :daily
    Rails.configuration.x.payment_processing.retries  # => 3
    Rails.configuration.x.super_debugger              # => true


  • Introduced Rails::Application.config_for to load a configuration for the current environment.

    # config/exception_notification.yml
      namespace: my_app_production
      url: http://localhost:3001
      namespace: my_app_development
    # config/environments/production.rb
    Rails.application.configure do
      config.middleware.use ExceptionNotifier, config_for(:exception_notification)

    (Pull Request)

  • Introduced a --skip-turbolinks option in the app generator to not generate turbolinks integration. (Commit)

  • Introduced a bin/setup script as a convention for automated setup code when bootstrapping an application. (Pull Request)

  • Changed the default value for config.assets.digest to true in development. (Pull Request)

  • Introduced an API to register new extensions for rake notes. (Pull Request)

  • Introduced an after_bundle callback for use in Rails templates. (Pull Request)

  • Introduced Rails.gem_version as a convenience method to return Gem::Version.new(Rails.version). (Pull Request)

5 Action Pack

Please refer to the Changelog for detailed changes.

5.1 Removals

  • respond_with and the class-level respond_to have been removed from Rails and moved to the responders gem (version 2.0). Add gem 'responders', '~> 2.0' to your Gemfile to continue using these features. (Pull Request, More Details)

  • Removed deprecated AbstractController::Helpers::ClassMethods::MissingHelperError in favor of AbstractController::Helpers::MissingHelperError. (Commit)

5.2 Deprecations

  • Deprecated the only_path option on *_path helpers. (Commit)

  • Deprecated assert_tag, assert_no_tag, find_tag and find_all_tag in favor of assert_select. (Commit)

  • Deprecated support for setting the :to option of a router to a symbol or a string that does not contain a "#" character:

    get '/posts', to: MyRackApp    => (No change necessary)
    get '/posts', to: 'post#index' => (No change necessary)
    get '/posts', to: 'posts'      => get '/posts', controller: :posts
    get '/posts', to: :index       => get '/posts', action: :index


  • Deprecated support for string keys in URL helpers:

    # bad
    root_path('controller' => 'posts', 'action' => 'index')
    # good
    root_path(controller: 'posts', action: 'index')

    (Pull Request)

5.3 Notable changes

  • The *_filter family of methods have been removed from the documentation. Their usage is discouraged in favor of the *_action family of methods:

    after_filter          => after_action
    append_after_filter   => append_after_action
    append_around_filter  => append_around_action
    append_before_filter  => append_before_action
    around_filter         => around_action
    before_filter         => before_action
    prepend_after_filter  => prepend_after_action
    prepend_around_filter => prepend_around_action
    prepend_before_filter => prepend_before_action
    skip_after_filter     => skip_after_action
    skip_around_filter    => skip_around_action
    skip_before_filter    => skip_before_action
    skip_filter           => skip_action_callback

    If your application currently depends on these methods, you should use the replacement *_action methods instead. These methods will be deprecated in the future and will eventually be removed from Rails.

    (Commit 1, 2)

  • render nothing: true or rendering a nil body no longer add a single space padding to the response body. (Pull Request)

  • Rails now automatically includes the template's digest in ETags. (Pull Request)

  • Segments that are passed into URL helpers are now automatically escaped. (Commit)

  • Introduced the always_permitted_parameters option to configure which parameters are permitted globally. The default value of this configuration is ['controller', 'action']. (Pull Request)

  • Added the HTTP method MKCALENDAR from RFC 4791. (Pull Request)

  • *_fragment.action_controller notifications now include the controller and action name in the payload. (Pull Request)

  • Improved the Routing Error page with fuzzy matching for route search. (Pull Request)

  • Added an option to disable logging of CSRF failures. (Pull Request)

  • When the Rails server is set to serve static assets, gzip assets will now be served if the client supports it and a pre-generated gzip file (.gz) is on disk. By default the asset pipeline generates .gz files for all compressible assets. Serving gzip files minimizes data transfer and speeds up asset requests. Always use a CDN if you are serving assets from your Rails server in production. (Pull Request)

  • When calling the process helpers in an integration test the path needs to have a leading slash. Previously you could omit it but that was a byproduct of the implementation and not an intentional feature, e.g.:

    test "list all posts" do
      get "/posts"
      assert_response :success

6 Action View

Please refer to the Changelog for detailed changes.

6.1 Deprecations

  • Deprecated AbstractController::Base.parent_prefixes. Override AbstractController::Base.local_prefixes when you want to change where to find views. (Pull Request)

  • Deprecated ActionView::Digestor#digest(name, format, finder, options = {}). Arguments should be passed as a hash instead. (Pull Request)

6.2 Notable changes

  • render "foo/bar" now expands to render template: "foo/bar" instead of render file: "foo/bar". (Pull Request)

  • The form helpers no longer generate a <div> element with inline CSS around the hidden fields. (Pull Request)

  • Introduced a #{partial_name}_iteration special local variable for use with partials that are rendered with a collection. It provides access to the current state of the iteration via the index, size, first? and last? methods. (Pull Request)

  • Placeholder I18n follows the same convention as label I18n. (Pull Request)

7 Action Mailer

Please refer to the Changelog for detailed changes.

7.1 Deprecations

  • Deprecated *_path helpers in mailers. Always use *_url helpers instead. (Pull Request)

  • Deprecated deliver / deliver! in favor of deliver_now / deliver_now!. (Pull Request)

7.2 Notable changes

  • link_to and url_for generate absolute URLs by default in templates, it is no longer needed to pass only_path: false. (Commit)

  • Introduced deliver_later which enqueues a job on the application's queue to deliver emails asynchronously. (Pull Request)

  • Added the show_previews configuration option for enabling mailer previews outside of the development environment. (Pull Request)

8 Active Record

Please refer to the Changelog for detailed changes.

8.1 Removals

  • Removed cache_attributes and friends. All attributes are cached. (Pull Request)

  • Removed deprecated method ActiveRecord::Base.quoted_locking_column. (Pull Request)

  • Removed deprecated ActiveRecord::Migrator.proper_table_name. Use the proper_table_name instance method on ActiveRecord::Migration instead. (Pull Request)

  • Removed unused :timestamp type. Transparently alias it to :datetime in all cases. Fixes inconsistencies when column types are sent outside of Active Record, such as for XML serialization. (Pull Request)

8.2 Deprecations

  • Deprecated swallowing of errors inside after_commit and after_rollback. (Pull Request)

  • Deprecated broken support for automatic detection of counter caches on has_many :through associations. You should instead manually specify the counter cache on the has_many and belongs_to associations for the through records. (Pull Request)

  • Deprecated passing Active Record objects to .find or .exists?. Call id on the objects first. (Commit 1, 2)

  • Deprecated half-baked support for PostgreSQL range values with excluding beginnings. We currently map PostgreSQL ranges to Ruby ranges. This conversion is not fully possible because Ruby ranges do not support excluded beginnings.

    The current solution of incrementing the beginning is not correct and is now deprecated. For subtypes where we don't know how to increment (e.g. succ is not defined) it will raise an ArgumentError for ranges with excluding beginnings. (Commit)

  • Deprecated calling DatabaseTasks.load_schema without a connection. Use DatabaseTasks.load_schema_current instead. (Commit)

  • Deprecated sanitize_sql_hash_for_conditions without replacement. Using a Relation for performing queries and updates is the preferred API. (Commit)

  • Deprecated add_timestamps and t.timestamps without passing the :null option. The default of null: true will change in Rails 5 to null: false. (Pull Request)

  • Deprecated Reflection#source_macro without replacement as it is no longer needed in Active Record. (Pull Request)

  • Deprecated serialized_attributes without replacement. (Pull Request)

  • Deprecated returning nil from column_for_attribute when no column exists. It will return a null object in Rails 5.0. (Pull Request)

  • Deprecated using .joins, .preload and .eager_load with associations that depend on the instance state (i.e. those defined with a scope that takes an argument) without replacement. (Commit)

8.3 Notable changes

  • SchemaDumper uses force: :cascade on create_table. This makes it possible to reload a schema when foreign keys are in place.

  • Added a :required option to singular associations, which defines a presence validation on the association. (Pull Request)

  • ActiveRecord::Dirty now detects in-place changes to mutable values. Serialized attributes on Active Record models are no longer saved when unchanged. This also works with other types such as string columns and json columns on PostgreSQL. (Pull Requests 1, 2, 3)

  • Introduced the db:purge Rake task to empty the database for the current environment. (Commit)

  • Introduced ActiveRecord::Base#validate! that raises ActiveRecord::RecordInvalid if the record is invalid. (Pull Request)

  • Introduced validate as an alias for valid?. (Pull Request)

  • touch now accepts multiple attributes to be touched at once. (Pull Request)

  • The PostgreSQL adapter now supports the jsonb datatype in PostgreSQL 9.4+. (Pull Request)

  • The PostgreSQL and SQLite adapters no longer add a default limit of 255 characters on string columns. (Pull Request)

  • Added support for the citext column type in the PostgreSQL adapter. (Pull Request)

  • Added support for user-created range types in the PostgreSQL adapter. (Commit)

  • sqlite3:///some/path now resolves to the absolute system path /some/path. For relative paths, use sqlite3:some/path instead. (Previously, sqlite3:///some/path resolved to the relative path some/path. This behavior was deprecated on Rails 4.1). (Pull Request)

  • Added support for fractional seconds for MySQL 5.6 and above. (Pull Request 1, 2)

  • Added ActiveRecord::Base#pretty_print to pretty print models. (Pull Request)

  • ActiveRecord::Base#reload now behaves the same as m = Model.find(m.id), meaning that it no longer retains the extra attributes from custom SELECTs. (Pull Request)

  • ActiveRecord::Base#reflections now returns a hash with string keys instead of symbol keys. (Pull Request)

  • The references method in migrations now supports a type option for specifying the type of the foreign key (e.g. :uuid). (Pull Request)

9 Active Model

Please refer to the Changelog for detailed changes.

9.1 Removals

  • Removed deprecated Validator#setup without replacement. (Pull Request)

9.2 Deprecations

  • Deprecated reset_#{attribute} in favor of restore_#{attribute}. (Pull Request)

  • Deprecated ActiveModel::Dirty#reset_changes in favor of clear_changes_information. (Pull Request)

9.3 Notable changes

  • Introduced validate as an alias for valid?. (Pull Request)

  • Introduced the restore_attributes method in ActiveModel::Dirty to restore the changed (dirty) attributes to their previous values. (Pull Request 1, 2)

  • has_secure_password no longer disallows blank passwords (i.e. passwords that contains only spaces) by default. (Pull Request)

  • has_secure_password now verifies that the given password is less than 72 characters if validations are enabled. (Pull Request)

10 Active Support

Please refer to the Changelog for detailed changes.

10.1 Removals

  • Removed deprecated Numeric#ago, Numeric#until, Numeric#since, Numeric#from_now. (Commit)

  • Removed deprecated string based terminators for ActiveSupport::Callbacks. (Pull Request)

10.2 Deprecations

  • Deprecated Kernel#silence_stderr, Kernel#capture and Kernel#quietly without replacement. (Pull Request)

  • Deprecated Class#superclass_delegating_accessor, use Class#class_attribute instead. (Pull Request)

  • Deprecated ActiveSupport::SafeBuffer#prepend! as ActiveSupport::SafeBuffer#prepend now performs the same function. (Pull Request)

10.3 Notable changes

  • Introduced a new configuration option active_support.test_order for specifying the order test cases are executed. This option currently defaults to :sorted but will be changed to :random in Rails 5.0. (Commit)

  • Object#try and Object#try! can now be used without an explicit receiver in the block. (Commit, Pull Request)

  • The travel_to test helper now truncates the usec component to 0. (Commit)

  • Introduced Object#itself as an identity function. (Commit 1, 2)

  • Object#with_options can now be used without an explicit receiver in the block. (Pull Request)

  • Introduced String#truncate_words to truncate a string by a number of words. (Pull Request)

  • Added Hash#transform_values and Hash#transform_values! to simplify a common pattern where the values of a hash must change, but the keys are left the same. (Pull Request)

  • The humanize inflector helper now strips any leading underscores. (Commit)

  • Introduced Concern#class_methods as an alternative to module ClassMethods, as well as Kernel#concern to avoid the module Foo; extend ActiveSupport::Concern; end boilerplate. (Commit)

  • New guide about constant autoloading and reloading.

11 Credits

See the full list of contributors to Rails for the many people who spent many hours making Rails the stable and robust framework it is today. Kudos to all of them.


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