rails--rails/guides/source/4_2_release_notes.md

**DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON https://guides.rubyonrails.org.**

Ruby on Rails 4.2 Release Notes
===============================

Highlights in Rails 4.2:

* Active Job
* Asynchronous mails
* Adequate Record
* Web Console
* Foreign key support

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](https://github.com/rails/rails/commits/4-2-stable) in
the main Rails repository on GitHub.

--------------------------------------------------------------------------------

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](upgrading_ruby_on_rails.html#upgrading-from-rails-4-1-to-rails-4-2).


Major Features
--------------

### Active Job

Active Job is a new framework in Rails 4.2. It is a common interface on top of
queuing systems like [Resque](https://github.com/resque/resque), [Delayed
Job](https://github.com/collectiveidea/delayed_job),
[Sidekiq](https://github.com/mperham/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](https://github.com/rails/globalid)
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:

```ruby
class TrashableCleanupJob < ActiveJob::Base
  def perform(trashable, depth)
    trashable.cleanup(depth)
  end
end
```

See the [Active Job Basics](active_job_basics.html) guide for more
information.

### 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`.

### 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](http://tenderlovemaking.com/2014/02/19/adequaterecord-pro-like-activerecord.html).

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:

```ruby
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')

post.comments
post.comments(true)
```

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.:

    ```ruby
    # not cached
    Post.find(1, 2, 3)
    Post.find([1,2])
    ```

- `find_by` with SQL fragments:

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

### Web Console

New applications generated with Rails 4.2 now come with the [Web
Console](https://github.com/rails/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.

### 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.

```ruby
# 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](https://api.rubyonrails.org/v4.2.0/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_foreign_key)
and
[remove_foreign_key](https://api.rubyonrails.org/v4.2.0/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-remove_foreign_key)
for a full description.


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.

### `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.

### `respond_with` / Class-Level `respond_to`

`respond_with` and the corresponding class-level `respond_to` have been moved
to the [responders](https://github.com/plataformatec/responders) gem. Add
`gem 'responders', '~> 2.0'` to your `Gemfile` to use it:

```ruby
# app/controllers/users_controller.rb

class UsersController < ApplicationController
  respond_to :html, :json

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

Instance-level `respond_to` is unaffected:

```ruby
# app/controllers/users_controller.rb

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

### Default Host for `rails server`

Due to a [change in Rack](https://github.com/rack/rack/commit/28b014484a8ac0bbb388e7eaeeef159598ec64fc),
`rails server` now listens on `localhost` instead of `0.0.0.0` by default. This
should have minimal impact on the standard development workflow as both
http://127.0.0.1:3000 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 0.0.0.0` 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.

### Changed status option symbols for `render`

Due to a [change in Rack](https://github.com/rack/rack/commit/be28c6a2ac152fe4adfbef71f3db9f4200df89e8), 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.

### HTML Sanitizer

The HTML sanitizer has been replaced with a new, more robust, implementation
built upon [Loofah](https://github.com/flavorjones/loofah) and
[Nokogiri](https://github.com/sparklemotion/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](https://github.com/kaspth/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](https://blog.plataformatec.com.br/2014/07/the-new-html-sanitizer-in-rails-4-2/)
for more details on the changes in the new sanitizer.

### `assert_select`

`assert_select` is now based on [Nokogiri](https://github.com/sparklemotion/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.

    ```ruby
    # before
    a[href=/]
    a[href$=/]

    # now
    a[href="/"]
    a[href$="/"]
    ```

*   DOMs built from HTML source containing invalid HTML with improperly
    nested elements may differ.

    For example:

    ```ruby
    # 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`).

    ```ruby
    # 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:

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

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

```ruby
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](https://github.com/rails/rails-dom-testing/tree/8798b9349fb9540ad8cb9a0ce6cb88d1384a210b) documentation for more on `assert_select`.


Railties
--------

Please refer to the [Changelog][railties] for detailed changes.

### Removals

*   The `--skip-action-view` option has been removed from the
    app generator. ([Pull Request](https://github.com/rails/rails/pull/17042))

*   The `rails application` command has been removed without replacement.
    ([Pull Request](https://github.com/rails/rails/pull/11616))

### Deprecations

*   Deprecated missing `config.log_level` for production environments.
    ([Pull Request](https://github.com/rails/rails/pull/16622))

*   Deprecated `rake test:all` in favor of `rake test` as it now run all tests
    in the `test` folder.
    ([Pull Request](https://github.com/rails/rails/pull/17348))

*   Deprecated `rake test:all:db` in favor of `rake test:db`.
    ([Pull Request](https://github.com/rails/rails/pull/17348))

*   Deprecated `Rails::Rack::LogTailer` without replacement.
    ([Commit](https://github.com/rails/rails/commit/84a13e019e93efaa8994b3f8303d635a7702dbce))

### Notable changes

*   Introduced `web-console` in the default application `Gemfile`.
    ([Pull Request](https://github.com/rails/rails/pull/11667))

*   Added a `required` option to the model generator for associations.
    ([Pull Request](https://github.com/rails/rails/pull/16062))

*   Introduced the `x` namespace for defining custom configuration options:

    ```ruby
    # 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:

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

    ([Commit](https://github.com/rails/rails/commit/611849772dd66c2e4d005dcfe153f7ce79a8a7db))

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

    ```yaml
    # config/exception_notification.yml
    production:
      url: http://127.0.0.1:8080
      namespace: my_app_production
    development:
      url: http://localhost:3001
      namespace: my_app_development
    ```

    ```ruby
    # config/environments/production.rb
    Rails.application.configure do
      config.middleware.use ExceptionNotifier, config_for(:exception_notification)
    end
    ```

    ([Pull Request](https://github.com/rails/rails/pull/16129))

*   Introduced a `--skip-turbolinks` option in the app generator to not generate
    turbolinks integration.
    ([Commit](https://github.com/rails/rails/commit/bf17c8a531bc8059d50ad731398002a3e7162a7d))

*   Introduced a `bin/setup` script as a convention for automated setup code when
    bootstrapping an application.
    ([Pull Request](https://github.com/rails/rails/pull/15189))

*   Changed the default value for `config.assets.digest` to `true` in development.
    ([Pull Request](https://github.com/rails/rails/pull/15155))

*   Introduced an API to register new extensions for `rake notes`.
    ([Pull Request](https://github.com/rails/rails/pull/14379))

*   Introduced an `after_bundle` callback for use in Rails templates.
    ([Pull Request](https://github.com/rails/rails/pull/16359))

*   Introduced `Rails.gem_version` as a convenience method to return
    `Gem::Version.new(Rails.version)`.
    ([Pull Request](https://github.com/rails/rails/pull/14101))


Action Pack
-----------

Please refer to the [Changelog][action-pack] for detailed changes.

### 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](https://github.com/rails/rails/pull/16526),
     [More Details](https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#responders))

*   Removed deprecated `AbstractController::Helpers::ClassMethods::MissingHelperError`
    in favor of `AbstractController::Helpers::MissingHelperError`.
    ([Commit](https://github.com/rails/rails/commit/a1ddde15ae0d612ff2973de9cf768ed701b594e8))

### Deprecations

*   Deprecated the `only_path` option on `*_path` helpers.
    ([Commit](https://github.com/rails/rails/commit/aa1fadd48fb40dd9396a383696134a259aa59db9))

*   Deprecated `assert_tag`, `assert_no_tag`, `find_tag` and `find_all_tag` in
    favor of `assert_select`.
    ([Commit](https://github.com/rails/rails-dom-testing/commit/b12850bc5ff23ba4b599bf2770874dd4f11bf750))

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

    ```ruby
    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
    ```

    ([Commit](https://github.com/rails/rails/commit/cc26b6b7bccf0eea2e2c1a9ebdcc9d30ca7390d9))

*   Deprecated support for string keys in URL helpers:

    ```ruby
    # bad
    root_path('controller' => 'posts', 'action' => 'index')

    # good
    root_path(controller: 'posts', action: 'index')
    ```

    ([Pull Request](https://github.com/rails/rails/pull/17743))

### 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](https://github.com/rails/rails/commit/6c5f43bab8206747a8591435b2aa0ff7051ad3de),
    [2](https://github.com/rails/rails/commit/489a8f2a44dc9cea09154ee1ee2557d1f037c7d4))

*   `render nothing: true` or rendering a `nil` body no longer add a single
    space padding to the response body.
    ([Pull Request](https://github.com/rails/rails/pull/14883))

*   Rails now automatically includes the template's digest in ETags.
    ([Pull Request](https://github.com/rails/rails/pull/16527))

*   Segments that are passed into URL helpers are now automatically escaped.
    ([Commit](https://github.com/rails/rails/commit/5460591f0226a9d248b7b4f89186bd5553e7768f))

*   Introduced the `always_permitted_parameters` option to configure which
    parameters are permitted globally. The default value of this configuration
    is `['controller', 'action']`.
    ([Pull Request](https://github.com/rails/rails/pull/15933))

*   Added the HTTP method `MKCALENDAR` from [RFC 4791](https://tools.ietf.org/html/rfc4791).
    ([Pull Request](https://github.com/rails/rails/pull/15121))

*   `*_fragment.action_controller` notifications now include the controller
    and action name in the payload.
    ([Pull Request](https://github.com/rails/rails/pull/14137))

*   Improved the Routing Error page with fuzzy matching for route search.
    ([Pull Request](https://github.com/rails/rails/pull/14619))

*   Added an option to disable logging of CSRF failures.
    ([Pull Request](https://github.com/rails/rails/pull/14280))

*   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](https://guides.rubyonrails.org/asset_pipeline.html#cdns) if you are
    serving assets from your Rails server in production.
    ([Pull Request](https://github.com/rails/rails/pull/16466))

*   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.:

    ```ruby
    test "list all posts" do
      get "/posts"
      assert_response :success
    end
    ```

Action View
-----------

Please refer to the [Changelog][action-view] for detailed changes.

### Deprecations

*   Deprecated `AbstractController::Base.parent_prefixes`.
    Override `AbstractController::Base.local_prefixes` when you want to change
    where to find views.
    ([Pull Request](https://github.com/rails/rails/pull/15026))

*   Deprecated `ActionView::Digestor#digest(name, format, finder, options = {})`.
    Arguments should be passed as a hash instead.
    ([Pull Request](https://github.com/rails/rails/pull/14243))

### Notable changes

*   `render "foo/bar"` now expands to `render template: "foo/bar"` instead of
    `render file: "foo/bar"`.
    ([Pull Request](https://github.com/rails/rails/pull/16888))

*   The form helpers no longer generate a `<div>` element with inline CSS around
    the hidden fields.
    ([Pull Request](https://github.com/rails/rails/pull/14738))

*   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](https://github.com/rails/rails/pull/7698))

*   Placeholder I18n follows the same convention as `label` I18n.
    ([Pull Request](https://github.com/rails/rails/pull/16438))


Action Mailer
-------------

Please refer to the [Changelog][action-mailer] for detailed changes.

### Deprecations

*   Deprecated `*_path` helpers in mailers. Always use `*_url` helpers instead.
    ([Pull Request](https://github.com/rails/rails/pull/15840))

*   Deprecated `deliver` / `deliver!` in favor of `deliver_now` / `deliver_now!`.
    ([Pull Request](https://github.com/rails/rails/pull/16582))

### 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](https://github.com/rails/rails/commit/9685080a7677abfa5d288a81c3e078368c6bb67c))

*   Introduced `deliver_later` which enqueues a job on the application's queue
    to deliver emails asynchronously.
    ([Pull Request](https://github.com/rails/rails/pull/16485))

*   Added the `show_previews` configuration option for enabling mailer previews
    outside of the development environment.
    ([Pull Request](https://github.com/rails/rails/pull/15970))


Active Record
-------------

Please refer to the [Changelog][active-record] for detailed changes.

### Removals

*   Removed `cache_attributes` and friends. All attributes are cached.
    ([Pull Request](https://github.com/rails/rails/pull/15429))

*   Removed deprecated method `ActiveRecord::Base.quoted_locking_column`.
    ([Pull Request](https://github.com/rails/rails/pull/15612))

*   Removed deprecated `ActiveRecord::Migrator.proper_table_name`. Use the
    `proper_table_name` instance method on `ActiveRecord::Migration` instead.
    ([Pull Request](https://github.com/rails/rails/pull/15512))

*   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](https://github.com/rails/rails/pull/15184))

### Deprecations

*   Deprecated swallowing of errors inside `after_commit` and `after_rollback`.
    ([Pull Request](https://github.com/rails/rails/pull/16537))

*   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](https://github.com/rails/rails/pull/15754))

*   Deprecated passing Active Record objects to `.find` or `.exists?`. Call
    `id` on the objects first.
    (Commit [1](https://github.com/rails/rails/commit/d92ae6ccca3bcfd73546d612efaea011270bd270),
    [2](https://github.com/rails/rails/commit/d35f0033c7dec2b8d8b52058fb8db495d49596f7))

*   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](https://github.com/rails/rails/commit/91949e48cf41af9f3e4ffba3e5eecf9b0a08bfc3))

*   Deprecated calling `DatabaseTasks.load_schema` without a connection. Use
    `DatabaseTasks.load_schema_current` instead.
    ([Commit](https://github.com/rails/rails/commit/f15cef67f75e4b52fd45655d7c6ab6b35623c608))

*   Deprecated `sanitize_sql_hash_for_conditions` without replacement. Using a
    `Relation` for performing queries and updates is the preferred API.
    ([Commit](https://github.com/rails/rails/commit/d5902c9e))

*   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](https://github.com/rails/rails/pull/16481))

*   Deprecated `Reflection#source_macro` without replacement as it is no longer
    needed in Active Record.
    ([Pull Request](https://github.com/rails/rails/pull/16373))

*   Deprecated `serialized_attributes` without replacement.
    ([Pull Request](https://github.com/rails/rails/pull/15704))

*   Deprecated returning `nil` from `column_for_attribute` when no column
    exists. It will return a null object in Rails 5.0.
    ([Pull Request](https://github.com/rails/rails/pull/15878))

*   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](https://github.com/rails/rails/commit/ed56e596a0467390011bc9d56d462539776adac1))

### 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](https://github.com/rails/rails/pull/16056))

*   `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](https://github.com/rails/rails/pull/15674),
    [2](https://github.com/rails/rails/pull/15786),
    [3](https://github.com/rails/rails/pull/15788))

*   Introduced the `db:purge` Rake task to empty the database for the
    current environment.
    ([Commit](https://github.com/rails/rails/commit/e2f232aba15937a4b9d14bd91e0392c6d55be58d))

*   Introduced `ActiveRecord::Base#validate!` that raises
    `ActiveRecord::RecordInvalid` if the record is invalid.
    ([Pull Request](https://github.com/rails/rails/pull/8639))

*   Introduced `validate` as an alias for `valid?`.
    ([Pull Request](https://github.com/rails/rails/pull/14456))

*   `touch` now accepts multiple attributes to be touched at once.
    ([Pull Request](https://github.com/rails/rails/pull/14423))

*   The PostgreSQL adapter now supports the `jsonb` datatype in PostgreSQL 9.4+.
    ([Pull Request](https://github.com/rails/rails/pull/16220))

*   The PostgreSQL and SQLite adapters no longer add a default limit of 255
    characters on string columns.
    ([Pull Request](https://github.com/rails/rails/pull/14579))

*   Added support for the `citext` column type in the PostgreSQL adapter.
    ([Pull Request](https://github.com/rails/rails/pull/12523))

*   Added support for user-created range types in the PostgreSQL adapter.
    ([Commit](https://github.com/rails/rails/commit/4cb47167e747e8f9dc12b0ddaf82bdb68c03e032))

*   `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](https://github.com/rails/rails/pull/14569))

*   Added support for fractional seconds for MySQL 5.6 and above.
    (Pull Request [1](https://github.com/rails/rails/pull/8240),
    [2](https://github.com/rails/rails/pull/14359))

*   Added `ActiveRecord::Base#pretty_print` to pretty print models.
    ([Pull Request](https://github.com/rails/rails/pull/15172))

*   `ActiveRecord::Base#reload` now behaves the same as `m = Model.find(m.id)`,
    meaning that it no longer retains the extra attributes from custom
    `SELECT`s.
    ([Pull Request](https://github.com/rails/rails/pull/15866))

*   `ActiveRecord::Base#reflections` now returns a hash with string keys instead
    of symbol keys. ([Pull Request](https://github.com/rails/rails/pull/17718))

*   The `references` method in migrations now supports a `type` option for
    specifying the type of the foreign key (e.g. `:uuid`).
    ([Pull Request](https://github.com/rails/rails/pull/16231))

Active Model
------------

Please refer to the [Changelog][active-model] for detailed changes.

### Removals

*   Removed deprecated `Validator#setup` without replacement.
    ([Pull Request](https://github.com/rails/rails/pull/10716))

### Deprecations

*   Deprecated `reset_#{attribute}` in favor of `restore_#{attribute}`.
    ([Pull Request](https://github.com/rails/rails/pull/16180))

*   Deprecated `ActiveModel::Dirty#reset_changes` in favor of
    `clear_changes_information`.
    ([Pull Request](https://github.com/rails/rails/pull/16180))

### Notable changes

*   Introduced `validate` as an alias for `valid?`.
    ([Pull Request](https://github.com/rails/rails/pull/14456))

*   Introduced the `restore_attributes` method in `ActiveModel::Dirty` to restore
    the changed (dirty) attributes to their previous values.
    (Pull Request [1](https://github.com/rails/rails/pull/14861),
    [2](https://github.com/rails/rails/pull/16180))

*   `has_secure_password` no longer disallows blank passwords (i.e. passwords
    that contains only spaces) by default.
    ([Pull Request](https://github.com/rails/rails/pull/16412))

*   `has_secure_password` now verifies that the given password is less than 72
    characters if validations are enabled.
    ([Pull Request](https://github.com/rails/rails/pull/15708))

Active Support
--------------

Please refer to the [Changelog][active-support] for detailed changes.

### Removals

*   Removed deprecated `Numeric#ago`, `Numeric#until`, `Numeric#since`,
    `Numeric#from_now`.
    ([Commit](https://github.com/rails/rails/commit/f1eddea1e3f6faf93581c43651348f48b2b7d8bb))

*   Removed deprecated string based terminators for `ActiveSupport::Callbacks`.
    ([Pull Request](https://github.com/rails/rails/pull/15100))

### Deprecations

*   Deprecated `Kernel#silence_stderr`, `Kernel#capture` and `Kernel#quietly`
    without replacement.
    ([Pull Request](https://github.com/rails/rails/pull/13392))

*   Deprecated `Class#superclass_delegating_accessor`, use
    `Class#class_attribute` instead.
    ([Pull Request](https://github.com/rails/rails/pull/14271))

*   Deprecated `ActiveSupport::SafeBuffer#prepend!` as
    `ActiveSupport::SafeBuffer#prepend` now performs the same function.
    ([Pull Request](https://github.com/rails/rails/pull/14529))

### 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](https://github.com/rails/rails/commit/53e877f7d9291b2bf0b8c425f9e32ef35829f35b))

*   `Object#try` and `Object#try!` can now be used without an explicit receiver in the block.
    ([Commit](https://github.com/rails/rails/commit/5e51bdda59c9ba8e5faf86294e3e431bd45f1830),
    [Pull Request](https://github.com/rails/rails/pull/17361))

*   The `travel_to` test helper now truncates the `usec` component to 0.
    ([Commit](https://github.com/rails/rails/commit/9f6e82ee4783e491c20f5244a613fdeb4024beb5))

*   Introduced `Object#itself` as an identity function.
    (Commit [1](https://github.com/rails/rails/commit/702ad710b57bef45b081ebf42e6fa70820fdd810),
    [2](https://github.com/rails/rails/commit/64d91122222c11ad3918cc8e2e3ebc4b0a03448a))

*   `Object#with_options` can now be used without an explicit receiver in the block.
    ([Pull Request](https://github.com/rails/rails/pull/16339))

*   Introduced `String#truncate_words` to truncate a string by a number of words.
    ([Pull Request](https://github.com/rails/rails/pull/16190))

*   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](https://github.com/rails/rails/pull/15819))

*   The `humanize` inflector helper now strips any leading underscores.
    ([Commit](https://github.com/rails/rails/commit/daaa21bc7d20f2e4ff451637423a25ff2d5e75c7))

*   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](https://github.com/rails/rails/commit/b16c36e688970df2f96f793a759365b248b582ad))

*   New [guide](autoloading_and_reloading_constants_classic_mode.html) about constant autoloading and reloading.

Credits
-------

See the
[full list of contributors to Rails](https://contributors.rubyonrails.org/) for
the many people who spent many hours making Rails the stable and robust
framework it is today. Kudos to all of them.

[railties]:       https://github.com/rails/rails/blob/4-2-stable/railties/CHANGELOG.md
[action-pack]:    https://github.com/rails/rails/blob/4-2-stable/actionpack/CHANGELOG.md
[action-view]:    https://github.com/rails/rails/blob/4-2-stable/actionview/CHANGELOG.md
[action-mailer]:  https://github.com/rails/rails/blob/4-2-stable/actionmailer/CHANGELOG.md
[active-record]:  https://github.com/rails/rails/blob/4-2-stable/activerecord/CHANGELOG.md
[active-model]:   https://github.com/rails/rails/blob/4-2-stable/activemodel/CHANGELOG.md
[active-support]: https://github.com/rails/rails/blob/4-2-stable/activesupport/CHANGELOG.md