Rails Mountable Engine - Useful starting point with rspec, factory girl and friends

15th February 2012 – 975 words

Since Ruby on Rails 3.1, engines were introduced. Unfortunatly, the generated code needs some love. I copied together some settings from various Stackoverflow posts regarding this topic.

Creating new engine

rails plugin new my_engine -T --mountable --dummy-path=spec/dummy

Creates a mountable (seperated) engine without test:unit. Now we can add some useful gems to my_engine.gemspec:

# let's bundler take care of our file list
  s.files        = `git ls-files`.split("\n")
  s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")

  s.add_dependency "rails", "~> 3.2"
  # s.add_dependency "jquery-rails"

  s.add_development_dependency "sqlite3"
  s.add_development_dependency "rspec-rails"
  s.add_development_dependency "shoulda-matchers", "~>1.0"#, "~> 3.0"
  s.add_development_dependency "factory_girl_rails"
  s.add_development_dependency "database_cleaner"
  # s.add_development_dependency "geminabox"
  # if you are using your own gem server, take geminabox

Configuring Rspec

Adding also the spec_helper-File for RSpec to work with our dummy app.

# Configure Rails Envinronment
ENV["RAILS_ENV"] = "test"
require File.expand_path("../dummy/config/environment.rb",  __FILE__)

require 'rspec/rails'
require "factory_girl_rails"
require "database_cleaner"
require 'shoulda/matchers/integrations/rspec'

ENGINE_RAILS_ROOT=File.join(File.dirname(__FILE__), '../')

# Requires supporting ruby files with custom matchers and macros, etc,
# in spec/support/ and its subdirectories.
Dir[File.join(ENGINE_RAILS_ROOT, "spec/support/**/*.rb")].each {|f| require f }

RSpec.configure do |config|
  config.use_transactional_fixtures = false
    config.before(:suite) do
      DatabaseCleaner.strategy = :transaction
    config.before(:each) do
    config.after(:each) do


Now we can use bundle exec rspec to run our tests from within the dummy app. Later, when we want to do integration with our real app, we should start by adding our gem to the app’s Gemfile via file-path. Doing so, we are able to work at our engine and viewing the results right in our development app (see later in this article).


Engines are IMO a very great idea. The engine’s directory structure is very similiar to a Rails dir structure. So placing all our stuff in the right place, will automatically load that files, e.g.:

  • my_engine/config/locales/en.yml
  • Cstom I18n strings
  • my_engine/app/controllers/models/my_engine/model1.rb
  • my_engine/db/migrate/123123123_migration1.rb
  • my_engine/app/assets/stylesheets/my_engine.css.scs
    • This stylesheet can be included in our real App with require my_engine inside the application.css manifest
  • …etc.

Namespacing everything

It is a good idea, to namespace the models, controllers etc. So make sure, the lib/feedmaker/engine.rb includes the isolated namespace:

module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine

Also, the models can be put into the same namespace, to avoid name collisions in the app later on.

# app/models/my_engine/model1.rb
class MyEngine::Model1< ActiveRecord::Base
  • Namespace the controller in the same way, putting everything under app/controllers/my_engine.
  • Same for views, which are located under app/views/my_engine/...


Add routes to config/routes in this way:

MyEngine::Engine.routes.draw do
  resources "feeds"
  root to: "feeds#index"

Due to our isolation, this resource will look for an feeds_controller inside the my_engine namespace.

Using the engine

Adding the gem to the Gemfile

# real applications Gemfile
#gem "my_engine"  # production
#gem "my_engine" ,path: '~/repos/my_engine'   # development
  • Copy over the migrations with
    rake my_engine:install:migrations

    and migrate.

  • Add assets to the manifest files, if required
  • Mount the engine to a specific URL:
# config/routes.rb
mount MyEngine::Engine => "/engine"
  • This will map the “/engine” url to the root-path from MyEngine.
  • Every other resource is also located under this namespace, e.g. the feeds_controller from above can be reached from “/engine/feeds”


  • The engine routes and url-helper are automatically namespaced to our engine’s namespaces, due to the isolated Engine directive. So do not use my_engine_model1_path, but model1_path in the engine’s views and controller.
  • An isolated mountable app normally render’s it own’s layout file (e.g. active admin has a own admin template). If you want to render from the Main-app’s template, change the app/controllers/my_engine/application_controller.rb, to inherit from the main app’s controller:
class MyEngine::ApplicationController < ApplicationController
  • If you are in the situation, that you use url-helpers in the app’s layout (or included partials), than you will get an error, that these helpers can not be found. Prefix the helper’s call with my_app -> link_to my_app.search_path


Looks a little bit messy, but after some setup everything works very nicely. Engines should be used, if there is a clearly distinguishable part of the app, which is independent from the rest. e.g. provision of assets, CMS-related features.

Reposted from notes.it-jobs-und-stellen.de