Proposed release notes for Rails 5, stems from PR #17227

5_0_release_notes.md

Ruby on Rails 5.0 Release Notes

Highlights in Rails 5.0:

• Explicitly halt callback chains

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.

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Upgrading to Rails 5.0

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 4.2 in case you haven't and make sure your application still runs as expected before attempting to upgrade to Rails 5.0. A list of things to watch out for when upgrading is available in the guide Upgrading Ruby on Rails.

Major Features

Explicitly halt callback chains

In Rails 4.2, when a 'before' callback returns false in ActiveRecord, ActiveModel and ActiveModel::Validations, then the entire callback chain is halted. In other words, successive 'before' callbacks are not executed, and neither is the action wrapped in callbacks.

class Post < ActiveRecord::Base
  before_save { |post| puts "Callback 1"; false }
  before_save { |post| puts "Callback 2" }
  after_save  { |post| puts "Callback 3" }
end

# Inside a new Rails 4.2 app
post = Post.new
post.save
# Callback 1
# => false

In Rails 5.0, returning false in a callback will not have the side effect of halting the callback chain.

# Inside a new Rails 5.0 app
post = Post.new
post.save
# Callback 1
# Callback 2
# Callback 3
# => true

When you upgrade an existing app from Rails 4.2 to Rails 5.0, you will still be able to halt callback chains by returning false, but you will receive a deprecation warning about this upcoming change.

# Inside a Rails 4.2 app upgraded to Rails 5.0
post = Post.new
post.save
# Callback 1
# DEPRECATION WARNING: Returning `false` in a callback will not implicitly halt a callback chain in the next release of Rails. To explicitly halt a callback chain, please use `throw :abort` instead.
# => false

Once you have replaced any return false with throw :abort, you can opt into the new behavior and remove the deprecation warning by adding the following configuration to your config/application.rb:

config.active_support.halt_callback_chains_on_return_false = false

See #17227 for more details.

Active Model

Deprecations

• Deprecated returning false as a way to halt ActiveModel and ActiveModel::Valdiations callback chains. The recommended way is to throw(:abort). (Pull Request)

Active Record

Deprecations

• Deprecated returning false as a way to halt ActiveRecord callback chains. The recommended way is to throw(:abort). (Pull Request)

Active Support

Notable changes

• New config option config.active_support.halt_callback_chains_on_return_false to specify whether ActiveRecord, ActiveModel and ActiveModel::Validations callback chains can be halted by returning false in a 'before' callback. (Pull Request)