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- What's this?
+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}[rdoc-ref:Rails::Railtie::rake_tasks] and {generators}[rdoc-ref:Rails::Railtie::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
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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]