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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, Action View and Active Record) is 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 any implementation of Railtie, but if you need to interact with the Rails framework during or after boot, then Railtie is needed.

For example, an extension doing any of the following would require 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 your Railtie

To extend Rails using Railtie, create a Railtie class which inherits from Rails::Railtie within your extension's namespace. This class must be loaded during the Rails boot process.

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 from your Railtie to Rails boot process, you just need to create an initializer block:

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 option to initializer, in case you want to couple it with a specific step in the initialization process.

Configuration

Inside the Railtie class, you 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 load 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

Application and Engine

A Rails::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
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Included Modules
Constants
ABSTRACT_RAILTIES = %w(Rails::Railtie Rails::Engine Rails::Application)
 
Class Public methods
abstract_railtie?()
# File railties/lib/rails/railtie.rb, line 159
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 181
def configure(&block)
  instance.configure(&block)
end
console(&blk)
# File railties/lib/rails/railtie.rb, line 141
def console(&blk)
  @load_console ||= []
  @load_console << blk if blk
  @load_console
end
generators(&blk)
# File railties/lib/rails/railtie.rb, line 153
def generators(&blk)
  @generators ||= []
  @generators << blk if blk
  @generators
end
inherited(base)
# File railties/lib/rails/railtie.rb, line 129
def inherited(base)
  unless base.abstract_railtie?
    subclasses << base
  end
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 170
def instance
  @instance ||= new
end
new()
# File railties/lib/rails/railtie.rb, line 203
def initialize
  if self.class.abstract_railtie?
    raise "#{self.class.name} is abstract, you cannot instantiate it directly."
  end
end
railtie_name(name = nil)
# File railties/lib/rails/railtie.rb, line 163
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 135
def rake_tasks(&blk)
  @rake_tasks ||= []
  @rake_tasks << blk if blk
  @rake_tasks
end
respond_to_missing?(*args)
# File railties/lib/rails/railtie.rb, line 174
def respond_to_missing?(*args)
  instance.respond_to?(*args) || super
end
runner(&blk)
# File railties/lib/rails/railtie.rb, line 147
def runner(&blk)
  @load_runner ||= []
  @load_runner << blk if blk
  @load_runner
end
subclasses()
# File railties/lib/rails/railtie.rb, line 125
def subclasses
  @subclasses ||= []
end
Class Protected methods
generate_railtie_name(string)
# File railties/lib/rails/railtie.rb, line 186
def generate_railtie_name(string)
  ActiveSupport::Inflector.underscore(string).tr("/", "_")
end
method_missing(name, *args, &block)

If the class method does not have a method, then send the method call to the Railtie instance.

# File railties/lib/rails/railtie.rb, line 192
def method_missing(name, *args, &block)
  if instance.respond_to?(name)
    instance.public_send(name, *args, &block)
  else
    super
  end
end
Instance Public methods
config()
# File railties/lib/rails/railtie.rb, line 213
def config
  @config ||= Railtie::Configuration.new
end
configure(&block)
# File railties/lib/rails/railtie.rb, line 209
def configure(&block)
  instance_eval(&block)
end
railtie_namespace()
# File railties/lib/rails/railtie.rb, line 217
def railtie_namespace
  @railtie_namespace ||= self.class.parents.detect { |n| n.respond_to?(:railtie_namespace) }
end