Rails::Engineallows you to wrap a specific Rails application or subset of functionality and share it with other applications. Since Rails 3.0, every Rails::Applicationis just an engine, which allows for simple feature and application sharing.

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

Creating an Engine

In Rails versions prior to 3.0, your gems automatically behaved as engines, however, this coupled Rails to Rubygems. Since Rails 3.0, if you want a gem to automatically behave as an engine, you have to specify an Enginefor it somewhere inside your plugin's libfolder (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

Besides the Railtieconfiguration which is shared across the application, in a Rails::Engineyou can access autoload_paths, eager_load_pathsand autoload_once_paths, which, differently from a Railtie, are scoped to the current engine.

Example:

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

  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

Since Rails 3.0, applications and engines have more flexible path configuration (as opposed to the previous hardcoded path configuration). This means 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/controllersand 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"]       # => ["config/routes.rb"]
end

The Applicationclass 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/observersfolder for example, it will be added by default.

Endpoint

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

To do that, use the endpointmethod:

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

Now you can mount your engine in application's routes just like that:

MyRailsApp::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
  match "/" => "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:

MyRailsApp::Application.routes.draw do
  mount MyEngine::Engine => "/blog"
  match "/blog/omg" => "main#omg"
end

MyEngineis mounted at /blog, and /blog/omgpoints to application's controller. In such a situation, requests to /blog/omgwill 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:

MyRailsApp::Application.routes.draw do
  match "/blog/omg" => "main#omg"
  mount MyEngine::Engine => "/blog"
end

Now, Enginewill 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

  • some of the rake tasks are based on engine name, e.g. my_engine:install:migrations, my_engine:install:assets

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

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 such controller:

module MyEngine
  class FooController < ActionController::Base
  end
end

If an engine is marked as isolated, FooControllerhas access only to helpers from Engineand url_helpersfrom 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 do namespace all your routes. With an isolated engine, the namespace is applied by default, so you can ignore it in routes:

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

The routes above will automatically point to MyEngine::ApplicationController. Furthermore, you don't need to use longer url helpers like my_engine_articles_path. Instead, you should simply use articles_pathas you would do with your application.

To make that behavior consistent with other parts of the framework, an isolated engine also has influence on ActiveModel::Naming. When you use a namespaced model, like MyEngine::Article, it will normally use the prefix “my_engine”. In an isolated engine, the prefix will be omitted in url helpers and form fields for convenience.

polymorphic_url(MyEngine::Article.new) # => "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 name according to namespace, so MyEngine::Engine.engine_name will be “my_engine”. It will also set MyEngine.table_name_prefix to “my_engine_”, changing the MyEngine::Article model to use the my_engine_articles table.

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_helpersinside 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
MyApplication::Application.routes.draw do
  mount MyEngine::Engine => "/my_engine", :as => "my_engine"
  match "/foo" => "foo#index"
end

Now, you can use the my_enginehelper inside your application:

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

There is also a main_apphelper 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 :asoption given to mount takes the engine_nameas 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 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 helpers 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 that 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 rake task, 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 seeds.rbfile. You can load that data using the load_seedmethod, e.g.

MyEngine::Engine.load_seed

Loading priority

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

Example:

# load Blog::Engine with highest priority, followed by application and other railties
config.railties_order = [Blog::Engine, :main_app, :all]
Namespace
Methods
#
A
C
D
E
F
H
I
L
O
R
Attributes
[RW] called_from
[RW] isolated
[RW] isolated?
Instance Public methods
app()
# File railties/lib/rails/engine.rb, line 472
def app
  @app ||= begin
    config.middleware = config.middleware.merge_into(default_middleware_stack)
    config.middleware.build(endpoint)
  end
end
call(env)
# File railties/lib/rails/engine.rb, line 483
def call(env)
  app.call(env.merge!(env_config))
end
config()
# File railties/lib/rails/engine.rb, line 515
def config
  @config ||= Engine::Configuration.new(find_root_with_flag("lib"))
end
eager_load!()
# File railties/lib/rails/engine.rb, line 438
def eager_load!
  railties.all(&:eager_load!)

  config.eager_load_paths.each do |load_path|
    matcher = /\A#{Regexp.escape(load_path)}\/(.*)\.rb\Z/
    Dir.glob("#{load_path}/**/*.rb").sort.each do |file|
      require_dependency file.sub(matcher, '\1')
    end
  end
end
endpoint(endpoint = nil)
# File railties/lib/rails/engine.rb, line 373
def endpoint(endpoint = nil)
  @endpoint ||= nil
  @endpoint = endpoint if endpoint
  @endpoint
end
env_config()
# File railties/lib/rails/engine.rb, line 487
def env_config
  @env_config ||= {
    'action_dispatch.routes' => routes
  }
end
find(path)

Finds engine with given path

# File railties/lib/rails/engine.rb, line 411
def find(path)
  expanded_path = File.expand_path path.to_s
  Rails::Engine::Railties.engines.find { |engine|
    File.expand_path(engine.root.to_s) == expanded_path
  }
end
helpers()
# File railties/lib/rails/engine.rb, line 453
def helpers
  @helpers ||= begin
    helpers = Module.new
    all = ActionController::Base.all_helpers_from_path(helpers_paths)
    ActionController::Base.modules_for_helpers(all).each do |mod|
      helpers.send(:include, mod)
    end
    helpers
  end
end
helpers_paths()
# File railties/lib/rails/engine.rb, line 464
def helpers_paths
  paths["app/helpers"].existent
end
inherited(base)
# File railties/lib/rails/engine.rb, line 361
def inherited(base)
  unless base.abstract_railtie?
    base.called_from = begin
      # Remove the line number from backtraces making sure we don't leave anything behind
      call_stack = caller.map { |p| p.sub(/:\d+.*/, '') }
      File.dirname(call_stack.detect { |p| p !~ %r[railties[\w.-]*/lib/rails|rack[\w.-]*/lib/rack] })
    end
  end

  super
end
initializers()
# File railties/lib/rails/engine.rb, line 503
def initializers
  initializers = []
  ordered_railties.each do |r|
    if r == self
      initializers += super
    else
      initializers += r.initializers
    end
  end
  initializers
end
isolate_namespace(mod)
# File railties/lib/rails/engine.rb, line 379
def isolate_namespace(mod)
  engine_name(generate_railtie_name(mod))

  self.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}_" }
      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) { railtie.routes_url_helpers }
      end
    end
  end
end
load_console(app=self)
# File railties/lib/rails/engine.rb, line 428
def load_console(app=self)
  railties.all { |r| r.load_console(app) }
  super
end
load_generators(app=self)
# File railties/lib/rails/engine.rb, line 348
def load_generators(app=self)
  initialize_generators
  railties.all { |r| r.load_generators(app) }
  Rails::Generators.configure!(app.config.generators)
  super
  self
end
load_runner(app=self)
# File railties/lib/rails/engine.rb, line 433
def load_runner(app=self)
  railties.all { |r| r.load_runner(app) }
  super
end
load_seed()

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

Blog::Engine.load_seed

# File railties/lib/rails/engine.rb, line 523
def load_seed
  seed_file = paths["db/seeds"].existent.first
  load(seed_file) if seed_file
end
load_tasks(app=self)
# File railties/lib/rails/engine.rb, line 422
def load_tasks(app=self)
  railties.all { |r| r.load_tasks(app) }
  super
  paths["lib/tasks"].existent.sort.each { |ext| load(ext) }
end
ordered_railties()
# File railties/lib/rails/engine.rb, line 499
def ordered_railties
  railties.all + [self]
end
railties()
# File railties/lib/rails/engine.rb, line 449
def railties
  @railties ||= self.class::Railties.new(config)
end
routes()
# File railties/lib/rails/engine.rb, line 493
def routes
  @routes ||= ActionDispatch::Routing::RouteSet.new
  @routes.append(&Proc.new) if block_given?
  @routes
end
routes_url_helpers()
# File railties/lib/rails/engine.rb, line 468
def routes_url_helpers
  routes.url_helpers
end
Instance Protected methods
_all_autoload_once_paths()
# File railties/lib/rails/engine.rb, line 650
def _all_autoload_once_paths
  config.autoload_once_paths
end
_all_autoload_paths()
# File railties/lib/rails/engine.rb, line 654
def _all_autoload_paths
  @_all_autoload_paths ||= (config.autoload_paths + config.eager_load_paths + config.autoload_once_paths).uniq
end
_all_load_paths()
# File railties/lib/rails/engine.rb, line 658
def _all_load_paths
  @_all_load_paths ||= (config.paths.load_paths + _all_autoload_paths).uniq
end
default_middleware_stack()
# File railties/lib/rails/engine.rb, line 646
def default_middleware_stack
  ActionDispatch::MiddlewareStack.new
end
find_root_with_flag(flag, default=nil)
# File railties/lib/rails/engine.rb, line 631
def find_root_with_flag(flag, default=nil)
  root_path = self.class.called_from

  while root_path && File.directory?(root_path) && !File.exist?("#{root_path}/#{flag}")
    parent = File.dirname(root_path)
    root_path = parent != root_path && parent
  end

  root = File.exist?("#{root_path}/#{flag}") ? root_path : default
  raise "Could not find root path for #{self}" unless root

  RbConfig::CONFIG['host_os'] =~ /mswin|mingw/ ?
    Pathname.new(root).expand_path : Pathname.new(root).realpath
end
has_migrations?()
# File railties/lib/rails/engine.rb, line 627
def has_migrations?
  paths["db/migrate"].existent.any?
end
initialize_generators()
# File railties/lib/rails/engine.rb, line 619
def initialize_generators
  require "rails/generators"
end
routes?()
# File railties/lib/rails/engine.rb, line 623
def routes?
  defined?(@routes)
end