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)

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.

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

Module

Class

Methods

Included Modules

Constants

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

Class Public methods

abstract_railtie?()

📝 Source code
# File railties/lib/rails/railtie.rb, line 155
      def abstract_railtie?
        ABSTRACT_RAILTIES.include?(name)
      end
🔎 See on GitHub

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.

📝 Source code
# File railties/lib/rails/railtie.rb, line 173
      def configure(&block)
        instance.configure(&block)
      end
🔎 See on GitHub

console(&blk)

📝 Source code
# File railties/lib/rails/railtie.rb, line 143
      def console(&blk)
        register_block_for(:load_console, &blk)
      end
🔎 See on GitHub

generators(&blk)

📝 Source code
# File railties/lib/rails/railtie.rb, line 151
      def generators(&blk)
        register_block_for(:generators, &blk)
      end
🔎 See on GitHub

inherited(base)

📝 Source code
# File railties/lib/rails/railtie.rb, line 133
      def inherited(base)
        unless base.abstract_railtie?
          subclasses << base
        end
      end
🔎 See on GitHub

instance()

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

📝 Source code
# File railties/lib/rails/railtie.rb, line 166
      def instance
        @instance ||= new
      end
🔎 See on GitHub

railtie_name(name = nil)

📝 Source code
# File railties/lib/rails/railtie.rb, line 159
      def railtie_name(name = nil)
        @railtie_name = name.to_s if name
        @railtie_name ||= generate_railtie_name(self.name)
      end
🔎 See on GitHub

rake_tasks(&blk)

📝 Source code
# File railties/lib/rails/railtie.rb, line 139
      def rake_tasks(&blk)
        register_block_for(:rake_tasks, &blk)
      end
🔎 See on GitHub

runner(&blk)

📝 Source code
# File railties/lib/rails/railtie.rb, line 147
      def runner(&blk)
        register_block_for(:runner, &blk)
      end
🔎 See on GitHub

subclasses()

📝 Source code
# File railties/lib/rails/railtie.rb, line 129
      def subclasses
        @subclasses ||= []
      end
🔎 See on GitHub

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.

📝 Source code
# File railties/lib/rails/railtie.rb, line 222
    def config
      @config ||= Railtie::Configuration.new
    end
🔎 See on GitHub