Quick overview of Rails engines
Engines are small Rails applications that can be used to add functionalities to the application hosting them. The class defining a Ruby on Rails application is
Rails::Application which actually inherits a lot of its behavior from
Rails::Engine, the class defining an engine. We can say that a regular Rails application is simply an engine with more features.
First, let’s generate a new Ruby on Rails application:
rails new ModularTodo
The next step is to generate an engine!
cd ModularTodo && rails plugin new todo --mountable
We will also create an ‘engines’ folder to store the engines (even if we just have one!).
mkdir engines && mv todo ./engines
Engines, just like gems, come with a gemspec file. Let’s put some real values to avoid warnings.
#ModularTodo/engines/todo/todo.gemspec $:.push File.expand_path("../lib", __FILE__) #Maintain your gem's version: require "todo/version" #Describe your gem and declare its dependencies: Gem::Specification.new do |s| s.name = "todo" s.version = Todo::VERSION s.authors = ["Thibault Denizet"] s.email = ["[email protected]"] s.homepage = "//samurails.com" s.summary = "Todo Module" s.description = "Todo Module for Modular Rails article" s.license = "MIT" #Moar stuff #... end
Now we need to add the Todo engine to the parent application Gemfile.
#ModularTodo/Gemfile #Other gems gem 'todo', path: 'engines/todo'
bundle install. You should see the following in the list of gems:
Using todo 0.0.1 from source at engines/todo
Great, our Todo engine is loaded correctly! Before we start coding, we have one last thing to do: mount the Todo engine. We can do that in the routes.rb file in the parent app.
Rails.application.routes.draw do mount Todo::Engine => "/", as: 'todo' end
We are mounting it at
/ but we could also make it accessible at
/todo. Since we have only one module,
/ is fine.
Now you can fire up your server and check it in your browser. You should see the default Rails view because we didn’t define any controllers/views yet. Let’s do that now!
We are going to scaffold a model named
Task inside the Todo module but to correctly migrate the database from the parent application, we need to add a small initializer to the
#ModularTodo/engines/todo/lib/todo/engine.rb module Todo class Engine < ::Rails::Engine isolate_namespace Todo initializer :append_migrations do |app| unless app.root.to_s.match(root.to_s) config.paths["db/migrate"].expanded.each do |p| app.config.paths["db/migrate"] << p end end end end end
That’s it, now when we run migrations from the parent application, the migrations in the Todo engine will be loaded too.
Let’s create the
Task model. The
scaffold command needs to be run from the engine folder.
cd engines/todo && rails g scaffold Task title:string content:text
Run the migrations from the parent folder:
Now, we just need to define the root route inside the Todo engine:
#ModularTodo/engines/todo/config/routes.rb Todo::Engine.routes.draw do resources :tasks root 'tasks#index' end
You can play with it, create tasks, delete them… Oh wait, the delete is not working! Why?! Well, it seems JQuery is not loaded, so let’s add it to the
application.js file inside the engine!
Yay, now we can destroy tasks!