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
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configuring a Rails framework for the application, like setting a generator
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+adding config.*+ keys to the environment
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setting up a subscriber with ActiveSupport::Notifications
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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.
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ABSTRACT_RAILTIES | = | %w(Rails::Railtie Rails::Engine Rails::Application) |