Ruby on RailsRails Engine - Modular Rails


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.


  • rails plugin new my_module --mountable

Create a modular app

# Getting started

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.

$:.push File.expand_path("../lib", __FILE__)

#Maintain your gem's version:
require "todo/version"

#Describe your gem and declare its dependencies: do |s|        = "todo"
  s.version     = Todo::VERSION
  s.authors     = ["Thibault Denizet"]       = ["[email protected]"]
  s.homepage    = "//"
  s.summary     = "Todo Module"
  s.description = "Todo Module for Modular Rails article"
  s.license     = "MIT"

  #Moar stuff

Now we need to add the Todo engine to the parent application Gemfile.

#Other gems
gem 'todo', path: 'engines/todo'

Let’s run 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'

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!

Building the Todo list

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 engine.rb file.

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


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:

rake db:migrate

Now, we just need to define the root route inside the Todo engine:

Todo::Engine.routes.draw do
  resources :tasks
  root 'tasks#index'

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!

// ModularTodo/engines/todo/app/assets/javascripts/todo/application.js
//= require jquery
//= require jquery_ujs
//= require_tree .

Yay, now we can destroy tasks!