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Rails::Railtie is the core of the Rails framework and provides several hooks to extend Rails and/or modify the initialization process.

Every major component of Rails (Action Mailer, Action Controller, Active Record, etc.) implements a railtie. Each of them is responsible for their own initialization. This makes Rails itself absent of any component hooks, allowing other components to be used in place of any of the Rails defaults.

Developing a Rails extension does not require implementing a railtie, but if you need to interact with the Rails framework during or after boot, then a railtie is needed.

For example, an extension doing any of the following would need a railtie:

  • creating initializers

  • configuring a Rails framework for the application, like setting a generator

  • adding config.* keys to the environment

  • setting up a subscriber with ActiveSupport::Notifications

  • adding Rake tasks

Creating a Railtie

To extend Rails using a railtie, create a subclass of Rails::Railtie. This class must be loaded during the Rails boot process, and is conventionally called MyNamespace::Railtie.

The following example demonstrates an extension which can be used with or without Rails.

# lib/my_gem/railtie.rb
module MyGem
  class Railtie < Rails::Railtie
  end
end

# lib/my_gem.rb
require "my_gem/railtie" if defined?(Rails::Railtie)

Initializers

To add an initialization step to the Rails boot process from your railtie, just define the initialization code with the initializer macro:

class MyRailtie < Rails::Railtie
  initializer "my_railtie.configure_rails_initialization" do
    # some initialization behavior
  end
end

If specified, the block can also receive the application object, in case you need to access some application-specific configuration, like middleware:

class MyRailtie < Rails::Railtie
  initializer "my_railtie.configure_rails_initialization" do |app|
    app.middleware.use MyRailtie::Middleware
  end
end

Finally, you can also pass :before and :after as options to initializer, in case you want to couple it with a specific step in the initialization process.

Configuration

Railties can access a config object which contains configuration shared by all railties and the application:

class MyRailtie < Rails::Railtie
  # Customize the ORM
  config.app_generators.orm :my_railtie_orm

  # Add a to_prepare block which is executed once in production
  # and before each request in development.
  config.to_prepare do
    MyRailtie.setup!
  end
end

Loading Rake Tasks and Generators

If your railtie has Rake tasks, you can tell Rails to load them through the method rake_tasks:

class MyRailtie < Rails::Railtie
  rake_tasks do
    load "path/to/my_railtie.tasks"
  end
end

By default, Rails loads generators from your load path. However, if you want to place your generators at a different location, you can specify in your railtie a block which will load them during normal generators lookup:

class MyRailtie < Rails::Railtie
  generators do
    require "path/to/my_railtie_generator"
  end
end

Since filenames on the load path are shared across gems, be sure that files you load through a railtie have unique names.

Run another program when the Rails server starts

In development, it's very usual to have to run another process next to the Rails Server. In example you might want to start the Webpack or React server. Or maybe you need to run your job scheduler process like Sidekiq. This is usually done by opening a new shell and running the program from here.

Rails allow you to specify a server block which will get called when a Rails server starts. This way, your users don't need to remember to have to open a new shell and run another program, making this less confusing for everyone. It can be used like this:

class MyRailtie < Rails::Railtie
  server do
    WebpackServer.start
  end
end

Application and Engine

An engine is nothing more than a railtie with some initializers already set. And since Rails::Application is an engine, the same configuration described here can be used in both.

Be sure to look at the documentation of those specific classes for more information.

Namespace
Methods
A
C
G
I
R
S
Included Modules

Constants

ABSTRACT_RAILTIES = %w(Rails::Railtie Rails::Engine Rails::Application)
 

Class Public methods

abstract_railtie?()

# File railties/lib/rails/railtie.rb, line 172
def abstract_railtie?
  ABSTRACT_RAILTIES.include?(name)
end

configure(&block)

Allows you to configure the railtie. This is the same method seen in Railtie::Configurable, but this module is no longer required for all subclasses of Railtie so we provide the class method here.

# File railties/lib/rails/railtie.rb, line 190
def configure(&block)
  instance.configure(&block)
end

console(&blk)

# File railties/lib/rails/railtie.rb, line 156
def console(&blk)
  register_block_for(:load_console, &blk)
end

generators(&blk)

# File railties/lib/rails/railtie.rb, line 164
def generators(&blk)
  register_block_for(:generators, &blk)
end

instance()

Since Rails::Railtie cannot be instantiated, any methods that call instance are intended to be called only on subclasses of a Railtie.

# File railties/lib/rails/railtie.rb, line 183
def instance
  @instance ||= new
end

railtie_name(name = nil)

# File railties/lib/rails/railtie.rb, line 176
def railtie_name(name = nil)
  @railtie_name = name.to_s if name
  @railtie_name ||= generate_railtie_name(self.name)
end

rake_tasks(&blk)

# File railties/lib/rails/railtie.rb, line 152
def rake_tasks(&blk)
  register_block_for(:rake_tasks, &blk)
end

runner(&blk)

# File railties/lib/rails/railtie.rb, line 160
def runner(&blk)
  register_block_for(:runner, &blk)
end

server(&blk)

# File railties/lib/rails/railtie.rb, line 168
def server(&blk)
  register_block_for(:server, &blk)
end

subclasses()

# File railties/lib/rails/railtie.rb, line 148
def subclasses
  super.reject(&:abstract_railtie?)
end

Instance Public methods

config()

This is used to create the config object on Railties, an instance of Railtie::Configuration, that is used by Railties and Application to store related configuration.

# File railties/lib/rails/railtie.rb, line 240
def config
  @config ||= Railtie::Configuration.new
end