Rails::Engine allows you to wrap a specific Rails application or subset of functionality and share it with other applications. Since Rails 3.0, every Rails::Application is just an engine, which allows for simple feature and application sharing.
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 Engine for it somewhere inside your plugin’s lib folder (similar to how we specify a Railtie):
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/*.
Besides the Railtie configuration which is shared across the application, in a Rails::Engine you can access autoload_paths, eager_load_paths and autoload_once_paths, which, differently from a Railtie, are scoped to the current engine.
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
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
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:
You can also have your controllers loaded from both app/controllers and lib/controllers:
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 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/observers folder for example, it will be added by default.
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 Engine and provide some of the Engine's features.
To do that, use the endpoint method:
Now you can mount your engine in application’s routes just like that:
MyRailsApp::Application.routes.draw do mount MyEngine::Engine => "/engine" end
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:
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
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:
MyRailsApp::Application.routes.draw do match "/blog/omg" => "main#omg" mount MyEngine::Engine => "/blog" end
Now, Engine will get only requests that were not handled by Application.
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::Engine it will be my_engine_engine. You can change it manually using the engine_name method:
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, FooController has access only to helpers from Engine 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 do namespace all your routes. With an isolated engine, the namespace is applied by default, so you can ignore it in routes:
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_path as 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_article 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_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 MyApplication::Application.routes.draw do mount MyEngine::Engine => "/my_engine", :as => "my_engine" match "/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:
This code will use my_engine.user_path(@user) to generate the proper route.
Isolated engine’s helpers
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:
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.rb file. You can load that data using the load_seed method, e.g.
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.
railties/lib/rails/engine/configuration.rb railties/lib/rails/engine/railties.rb railties/lib/rails/engine.rb