Question

Can someone help me understand the differences between a Rails Engine and a Mountable app? In Rails 3.1, you can create either one with the “rails new plugin ___” command.

rails plugin new forum --full        # Engine
rails plugin new forum --mountable   # Mountable App

When would you want to use one versus the other? I know you can package an Engine as a gem, for one. Is that not the case for Mountable Apps? What other differences are there?

Answer 1

I have noticed the following:

Full Engine

With a full engine, the parent application inherits the routes from the engine. It is not necessary to specify anything in parent_app/config/routes.rb. Specifying the gem in Gemfile is enough for the parent app to inherit the models, routes etc. The engine routes are specified as:

# my_engine/config/routes.rb 
Rails.application.routes.draw do 
  # whatever 
end 

No namespacing of models, controllers, etc. These are immediately
accessible to the parent application.

Mountable Engine

The engine’s namespace is isolated by default:

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

With a mountable engine, the routes are namespaced and the parent app can bundle this functionality under a single route:

# my_engine/config/routes.rb 
MyEngine::Engine.routes.draw do 
  #whatever 
end 

# parent_app/config/routes.rb 
ParentApp::Application.routes.draw do 
    mount MyEngine::Engine => "/engine", :as => "namespaced" 
end 

Models, controllers, etc are isolated from the parent application – although helpers can be shared easily.

These are the main differences I have spotted. Perhaps there are others? I have asked over here, but have yet to receive a response.

My impression is that since a full engine does not isolate itself from the parent application, it is best used as a standalone application adjacent to the parent app. I believe name clashes could occur.

A mountable engine could be used in situations where you want to avoid name conflicts and bundle the engine under one specific route in the parent application. For example, I am working on building my first engine designed for customer service. The parent application could bundle it’s functionality under a single route such as:

mount Cornerstone::Engine => "/cornerstone", :as => "help" 

If I’m way off in my assumptions, someone please let me know and I’ll fix this response. I have made a small article about the subject here.