Rails::Engine allows you to wrap a specific Rails application or subset of functionality and share it with other applications or within a larger packaged application. Every Rails::Application is just an engine, which allows for simple feature and application sharing.

Any Rails::Engine is also a Rails::Railtie, so the same methods (like rake_tasks and generators) and configuration options that are available in railties can also be used in engines.

Creating an Engine

If you want a gem to behave as an engine, you have to specify an Engine for it somewhere inside your plugin’s lib folder (similar to how we specify a Railtie):

# lib/my_engine.rb
module MyEngine
  class Engine < Rails::Engine
  end
end

Then ensure that this file is loaded at the top of your config/application.rb (or in your Gemfile), and it will automatically load models, controllers, and helpers inside app, load routes at config/routes.rb, load locales at config/locales/*/, and load tasks at lib/tasks/*/.

Configuration

Like railties, engines can access a config object which contains configuration shared by all railties and the application. Additionally, each engine can access autoload_paths, eager_load_paths and autoload_once_paths settings which are scoped to that engine.

class MyEngine < Rails::Engine
  # Add a load path for this specific Engine
  config.autoload_paths << File.expand_path("lib/some/path", __dir__)

  initializer "my_engine.add_middleware" do |app|
    app.middleware.use MyEngine::Middleware
  end
end

Generators

You can set up generators for engines with config.generators method:

class MyEngine < Rails::Engine
  config.generators do |g|
    g.orm             :active_record
    g.template_engine :erb
    g.test_framework  :test_unit
  end
end

You can also set generators for an application by using config.app_generators:

class MyEngine < Rails::Engine
  # note that you can also pass block to app_generators in the same way you
  # can pass it to generators method
  config.app_generators.orm :datamapper
end

Paths

Applications and engines have flexible path configuration, meaning that you are not required to place your controllers at app/controllers, but in any place which you find convenient.

For example, let’s suppose you want to place your controllers in lib/controllers. You can set that as an option:

class MyEngine < Rails::Engine
  paths["app/controllers"] = "lib/controllers"
end

You can also have your controllers loaded from both app/controllers and lib/controllers:

class MyEngine < Rails::Engine
  paths["app/controllers"] << "lib/controllers"
end

The available paths in an engine are:

class MyEngine < Rails::Engine
  paths["app"]                 # => ["app"]
  paths["app/controllers"]     # => ["app/controllers"]
  paths["app/helpers"]         # => ["app/helpers"]
  paths["app/models"]          # => ["app/models"]
  paths["app/views"]           # => ["app/views"]
  paths["lib"]                 # => ["lib"]
  paths["lib/tasks"]           # => ["lib/tasks"]
  paths["config"]              # => ["config"]
  paths["config/initializers"] # => ["config/initializers"]
  paths["config/locales"]      # => ["config/locales"]
  paths["config/routes.rb"]    # => ["config/routes.rb"]
end

The Application class adds a couple more paths to this set. And as in your Application, all folders under app are automatically added to the load path. If you have an app/services folder for example, it will be added by default.

Endpoint

An engine can also be a Rack application. It can be useful if you have a Rack application that you would like to provide with some of the Engine‘s features.

To do that, use the ::endpoint method:

module MyEngine
  class Engine < Rails::Engine
    endpoint MyRackApplication
  end
end

Now you can mount your engine in application’s routes:

Rails.application.routes.draw do
  mount MyEngine::Engine => "/engine"
end

Middleware stack

As an engine can now be a Rack endpoint, it can also have a middleware stack. The usage is exactly the same as in Application:

module MyEngine
  class Engine < Rails::Engine
    middleware.use SomeMiddleware
  end
end

Routes

If you don’t specify an endpoint, routes will be used as the default endpoint. You can use them just like you use an application’s routes:

# ENGINE/config/routes.rb
MyEngine::Engine.routes.draw do
  get "/" => "posts#index"
end

Mount priority

Note that now there can be more than one router in your application, and it’s better to avoid passing requests through many routers. Consider this situation:

Rails.application.routes.draw do
  mount MyEngine::Engine => "/blog"
  get "/blog/omg" => "main#omg"
end

MyEngine is mounted at /blog, and /blog/omg points to application’s controller. In such a situation, requests to /blog/omg will go through MyEngine, and if there is no such route in Engine‘s routes, it will be dispatched to main#omg. It’s much better to swap that:

Rails.application.routes.draw do
  get "/blog/omg" => "main#omg"
  mount MyEngine::Engine => "/blog"
end

Now, Engine will get only requests that were not handled by Application.

Engine name

There are some places where an Engine’s name is used:

  • routes: when you mount an Engine with mount(MyEngine::Engine => '/my_engine'), it’s used as default :as option

  • rake task for installing migrations my_engine:install:migrations

Engine name is set by default based on class name. For MyEngine::Engine it will be my_engine_engine. You can change it manually using the engine_name method:

module MyEngine
  class Engine < Rails::Engine
    engine_name "my_engine"
  end
end

Isolated Engine

Normally when you create controllers, helpers, and models inside an engine, they are treated as if they were created inside the application itself. This means that all helpers and named routes from the application will be available to your engine’s controllers as well.

However, sometimes you want to isolate your engine from the application, especially if your engine has its own router. To do that, you simply need to call ::isolate_namespace. This method requires you to pass a module where all your controllers, helpers, and models should be nested to:

module MyEngine
  class Engine < Rails::Engine
    isolate_namespace MyEngine
  end
end

With such an engine, everything that is inside the MyEngine module will be isolated from the application.

Consider this controller:

module MyEngine
  class FooController < ActionController::Base
  end
end

If the MyEngine engine is marked as isolated, FooController only has access to helpers from MyEngine, and url_helpers from MyEngine::Engine.routes.

The next thing that changes in isolated engines is the behavior of routes. Normally, when you namespace your controllers, you also need to namespace the related routes. With an isolated engine, the engine’s namespace is automatically applied, so you don’t need to specify it explicitly in your routes:

MyEngine::Engine.routes.draw do
  resources :articles
end

If MyEngine is isolated, the routes above will point to MyEngine::ArticlesController. You also don’t need to use longer URL helpers like my_engine_articles_path. Instead, you should simply use articles_path, like you would do with your main application.

To make this behavior consistent with other parts of the framework, isolated engines also have an effect on ActiveModel::Naming. In a normal Rails app, when you use a namespaced model such as Namespace::Article, ActiveModel::Naming will generate names with the prefix “namespace”. In an isolated engine, the prefix will be omitted in URL helpers and form fields, for convenience.

polymorphic_url(MyEngine::Article.new)
# => "articles_path" # not "my_engine_articles_path"

form_for(MyEngine::Article.new) do
  text_field :title # => <input type="text" name="article[title]" id="article_title" />
end

Additionally, an isolated engine will set its own name according to its namespace, so MyEngine::Engine.engine_name will return “my_engine”. It will also set MyEngine.table_name_prefix to “my_engine_”, meaning for example that MyEngine::Article will use the my_engine_articles database table by default.

Using Engine’s routes outside Engine

Since you can now mount an engine inside application’s routes, you do not have direct access to Engine‘s url_helpers inside Application. When you mount an engine in an application’s routes, a special helper is created to allow you to do that. Consider such a scenario:

# config/routes.rb
Rails.application.routes.draw do
  mount MyEngine::Engine => "/my_engine", as: "my_engine"
  get "/foo" => "foo#index"
end

Now, you can use the my_engine helper inside your application:

class FooController < ApplicationController
  def index
    my_engine.root_url # => /my_engine/
  end
end

There is also a main_app helper that gives you access to application’s routes inside Engine:

module MyEngine
  class BarController
    def index
      main_app.foo_path # => /foo
    end
  end
end

Note that the :as option given to mount takes the engine_name as default, so most of the time you can simply omit it.

Finally, if you want to generate a URL to an engine’s route using polymorphic_url, you also need to pass the engine helper. Let’s say that you want to create a form pointing to one of the engine’s routes. All you need to do is pass the helper as the first element in array with attributes for URL:

form_for([my_engine, @user])

This code will use my_engine.user_path(@user) to generate the proper route.

Isolated engine’s helpers

Sometimes you may want to isolate an engine, but use helpers that are defined for it. If you want to share just a few specific helpers you can add them to application’s helpers in ApplicationController:

class ApplicationController < ActionController::Base
  helper MyEngine::SharedEngineHelper
end

If you want to include all of the engine’s helpers, you can use the helper method on an engine’s instance:

class ApplicationController < ActionController::Base
  helper MyEngine::Engine.helpers
end

It will include all of the helpers from engine’s directory. Take into account this does not include helpers defined in controllers with helper_method or other similar solutions, only helpers defined in the helpers directory will be included.

Migrations & seed data

Engines can have their own migrations. The default path for migrations is exactly the same as in application: db/migrate

To use engine’s migrations in application you can use the rake task below, which copies them to application’s dir:

$ rake ENGINE_NAME:install:migrations

Note that some of the migrations may be skipped if a migration with the same name already exists in application. In such a situation you must decide whether to leave that migration or rename the migration in the application and rerun copying migrations.

If your engine has migrations, you may also want to prepare data for the database in the db/seeds.rb file. You can load that data using the load_seed method, e.g.

MyEngine::Engine.load_seed

Loading priority

In order to change engine’s priority you can use config.railties_order in the main application. It will affect the priority of loading views, helpers, assets, and all the other files related to engine or application.

# load Blog::Engine with highest priority, followed by application and other railties
config.railties_order = [Blog::Engine, :main_app, :all]

Namespace

Class

Methods

Included Modules

Attributes

[RW] called_from
[RW] isolated
[RW] isolated?

Class Public methods

endpoint(endpoint = nil)

📝 Source code
# File railties/lib/rails/engine.rb, line 379
      def endpoint(endpoint = nil)
        @endpoint ||= nil
        @endpoint = endpoint if endpoint
        @endpoint
      end
🔎 See on GitHub

find(path)

Finds engine with given path.

📝 Source code
# File railties/lib/rails/engine.rb, line 423
      def find(path)
        expanded_path = File.expand_path path
        Rails::Engine.subclasses.each do |klass|
          engine = klass.instance
          return engine if File.expand_path(engine.root) == expanded_path
        end
        nil
      end
🔎 See on GitHub

find_root(from)

📝 Source code
# File railties/lib/rails/engine.rb, line 375
      def find_root(from)
        find_root_with_flag "lib", from
      end
🔎 See on GitHub

inherited(base)

📝 Source code
# File railties/lib/rails/engine.rb, line 361
      def inherited(base)
        unless base.abstract_railtie?
          Rails::Railtie::Configuration.eager_load_namespaces << base

          base.called_from = begin
            call_stack = caller_locations.map { |l| l.absolute_path || l.path }

            File.dirname(call_stack.detect { |p| !p.match?(%r[railties[\w.-]*/lib/rails|rack[\w.-]*/lib/rack]) })
          end
        end

        super
      end
🔎 See on GitHub

isolate_namespace(mod)

📝 Source code
# File railties/lib/rails/engine.rb, line 385
      def isolate_namespace(mod)
        engine_name(generate_railtie_name(mod.name))

        routes.default_scope = { module: ActiveSupport::Inflector.underscore(mod.name) }
        self.isolated = true

        unless mod.respond_to?(:railtie_namespace)
          name, railtie = engine_name, self

          mod.singleton_class.instance_eval do
            define_method(:railtie_namespace) { railtie }

            unless mod.respond_to?(:table_name_prefix)
              define_method(:table_name_prefix) { "#{name}_" }

              ActiveSupport.on_load(:active_record) do
                mod.singleton_class.redefine_method(:table_name_prefix) do
                  "#{ActiveRecord::Base.table_name_prefix}#{name}_"
                end
              end
            end

            unless mod.respond_to?(:use_relative_model_naming?)
              class_eval "def use_relative_model_naming?; true; end", __FILE__, __LINE__
            end

            unless mod.respond_to?(:railtie_helpers_paths)
              define_method(:railtie_helpers_paths) { railtie.helpers_paths }
            end

            unless mod.respond_to?(:railtie_routes_url_helpers)
              define_method(:railtie_routes_url_helpers) { |include_path_helpers = true| railtie.routes.url_helpers(include_path_helpers) }
            end
          end
        end
      end
🔎 See on GitHub

new()

📝 Source code
# File railties/lib/rails/engine.rb, line 439
    def initialize
      @_all_autoload_paths = nil
      @_all_load_paths     = nil
      @app                 = nil
      @config              = nil
      @env_config          = nil
      @helpers             = nil
      @routes              = nil
      @app_build_lock      = Mutex.new
      super
    end
🔎 See on GitHub

Instance Public methods

app()

Returns the underlying Rack application for this engine.

📝 Source code
# File railties/lib/rails/engine.rb, line 517
    def app
      @app || @app_build_lock.synchronize {
        @app ||= begin
          stack = default_middleware_stack
          config.middleware = build_middleware.merge_into(stack)
          config.middleware.build(endpoint)
        end
      }
    end
🔎 See on GitHub

call(env)

Define the Rack API for this engine.

📝 Source code
# File railties/lib/rails/engine.rb, line 534
    def call(env)
      req = build_request env
      app.call req.env
    end
🔎 See on GitHub

config()

Define the configuration object for the engine.

📝 Source code
# File railties/lib/rails/engine.rb, line 553
    def config
      @config ||= Engine::Configuration.new(self.class.find_root(self.class.called_from))
    end
🔎 See on GitHub

eager_load!()

📝 Source code
# File railties/lib/rails/engine.rb, line 491
    def eager_load!
      # Already done by Zeitwerk::Loader.eager_load_all. By now, we leave the
      # method as a no-op for backwards compatibility.
    end
🔎 See on GitHub

endpoint()

Returns the endpoint for this engine. If none is registered, defaults to an ActionDispatch::Routing::RouteSet.

📝 Source code
# File railties/lib/rails/engine.rb, line 529
    def endpoint
      self.class.endpoint || routes
    end
🔎 See on GitHub

env_config()

Defines additional Rack env configuration that is added on each call.

📝 Source code
# File railties/lib/rails/engine.rb, line 540
    def env_config
      @env_config ||= {}
    end
🔎 See on GitHub

helpers()

Returns a module with all the helpers defined for the engine.

📝 Source code
# File railties/lib/rails/engine.rb, line 501
    def helpers
      @helpers ||= begin
        helpers = Module.new
        AbstractController::Helpers.helper_modules_from_paths(helpers_paths).each do |mod|
          helpers.include(mod)
        end
        helpers
      end
    end
🔎 See on GitHub

helpers_paths()

Returns all registered helpers paths.

📝 Source code
# File railties/lib/rails/engine.rb, line 512
    def helpers_paths
      paths["app/helpers"].existent
    end
🔎 See on GitHub

load_console(app = self)

Load console and invoke the registered hooks. Check Rails::Railtie.console for more info.

📝 Source code
# File railties/lib/rails/engine.rb, line 453
    def load_console(app = self)
      require "rails/console/app"
      require "rails/console/helpers"
      run_console_blocks(app)
      self
    end
🔎 See on GitHub

load_generators(app = self)

Load Rails generators and invoke the registered hooks. Check Rails::Railtie.generators for more info.

📝 Source code
# File railties/lib/rails/engine.rb, line 477
    def load_generators(app = self)
      require "rails/generators"
      run_generators_blocks(app)
      Rails::Generators.configure!(app.config.generators)
      self
    end
🔎 See on GitHub

load_runner(app = self)

Load Rails runner and invoke the registered hooks. Check Rails::Railtie.runner for more info.

📝 Source code
# File railties/lib/rails/engine.rb, line 462
    def load_runner(app = self)
      run_runner_blocks(app)
      self
    end
🔎 See on GitHub

load_seed()

Load data from db/seeds.rb file. It can be used in to load engines’ seeds, e.g.:

Blog::Engine.load_seed

📝 Source code
# File railties/lib/rails/engine.rb, line 561
    def load_seed
      seed_file = paths["db/seeds.rb"].existent.first
      run_callbacks(:load_seed) { load(seed_file) } if seed_file
    end
🔎 See on GitHub

load_server(app = self)

Invoke the server registered hooks. Check Rails::Railtie.server for more info.

📝 Source code
# File railties/lib/rails/engine.rb, line 486
    def load_server(app = self)
      run_server_blocks(app)
      self
    end
🔎 See on GitHub

load_tasks(app = self)

Load Rake and railties tasks, and invoke the registered hooks. Check Rails::Railtie.rake_tasks for more info.

📝 Source code
# File railties/lib/rails/engine.rb, line 469
    def load_tasks(app = self)
      require "rake"
      run_tasks_blocks(app)
      self
    end
🔎 See on GitHub

railties()

📝 Source code
# File railties/lib/rails/engine.rb, line 496
    def railties
      @railties ||= Railties.new
    end
🔎 See on GitHub

routes(&block)

Defines the routes for this engine. If a block is given to routes, it is appended to the engine.

📝 Source code
# File railties/lib/rails/engine.rb, line 546
    def routes(&block)
      @routes ||= ActionDispatch::Routing::RouteSet.new_with_config(config)
      @routes.append(&block) if block_given?
      @routes
    end
🔎 See on GitHub

Instance Private methods

load_config_initializer(initializer)

📝 Source code
# File railties/lib/rails/engine.rb, line 688
      def load_config_initializer(initializer) # :doc:
        ActiveSupport::Notifications.instrument("load_config_initializer.railties", initializer: initializer) do
          load(initializer)
        end
      end
🔎 See on GitHub