A Guide for Upgrading to Ruby on Rails 4.2.0

As you might have already heard, the latest stuff for upgrading rails was finally released at the end of the last year, viz. 4.2.0 was released on Dec 19, 2014. Well, here is the quick overview of its major tools:

Active Job 

It is a common interface on top of queuing systems like Resque, Delayed Job, Sidekiq, etc.

Active Job is a framework for declaring jobs and making them run on a variety of queueing backends. These jobs can be everything from regularly scheduled clean-ups, to billing charges, to mailings; anything that can be separated into small units of work and run in parallel.

Also there could be other framework features and other gems built on top of that, without having to worry about API differences between Delayed Job and Resque.

Usage

Set the queue adapter for Active Job:

ActiveJob::Base.queue_adapter = :inline # default queue adapter

Declare a job like so:

classMyJob< ActiveJob::Base
  queue_as :my_jobs
  defperform(record)
    record.do_work
  end
end

Enqueue a job like so:

MyJob.perform_later record  # Enqueue a job to be performed as soon the queueing system is free.
MyJob.set(wait_until: Date.tomorrow.noon).perform_later(record)  # Enqueue a job to be performed tomorrow at noon.
MyJob.set(wait: 1.week).perform_later(record) # Enqueue a job to be performed 1 week from now.

That's all.

GlobalID support

Tip:

A Global ID is a URI that uniquely identifies a model instance:

gid://YourApp/Some::Model/id

This is helpful when you need a single identifier to reference different classes of objects.

One example is job scheduling:

we need to reference a model object rather than serialize the object itself. We can pass a Global ID that can be used to locate the model when it's time to perform the job. The job scheduler doesn't need to know the details of model naming and IDs, just that it has a global identifier that references a model.

Another example:

This makes it possible to pass live Active Record objects to your job instead of class / id pairs, which you then have to manually deserialize. Before, jobs would look like this:

classTrashableCleanupJob
  defperform(trashable_class, trashable_id, depth)
    trashable = trashable_class.constantize.find(trashable_id)
    trashable.cleanup(depth)
  end
end

 Now you can simply do:

classTrashableCleanupJob
  defperform(trashable, depth)
    trashable.cleanup(depth)
  end
end

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)

group :development do
  gem 'web-console', '~> 2.0'
end

classApplication< Rails::Application
  config.web_console.whitelisted_ips = %w( 127.0.0.1 192.168.0.100 )
end

classApplication< Rails::Application
  config.web_console.whitelisted_ips = '192.168.0.0/16'
end

# 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

Incompatibilities

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.

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
classUsersController< ApplicationController
  respond_to :html, :json
 
  defshow
    @user = User.find(params[:id])
    respond_with @user
  end
end

Instance-level respond_to is unaffected:

# app/controllers/users_controller.rb
classUsersController< ApplicationController
  defshow
    @user = User.find(params[:id])
    respond_to do |format|
      format.html
      format.json { render json: @user }
    end
  end
end

3. Default Host for rails server

Due to a change in Rack, 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.

4. HTML Sanitizer

The HTML sanitizer has been replaced with a new, more robust, implementation built upon Loofahand 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.

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

5. 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:

# 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:

# 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&T), and now is evaluated (e.g. AT&T).

# content: <p>AT&amp;T</p>

# before:
assert_select('p', 'AT&T')  # => true
assert_select('p', 'AT&T')      # => false
 
# now:
assert_select('p', 'AT&T')      # => true
assert_select('p', 'AT&T')  # => false

There are hundreds of posts on this topic. So please feel free to share others in the comments, to make this resource list longer and better.