Single-file Rails 3.2 Application

Creating a Rails 3.2 application in a single file for testing a gem.

Update 2013/05/27: configure Rails.root for the application.


Gems often provide functionality specific to Rails applications, without necessarily depending on Rails themselves. To properly test these gems, we need a Rails application using it. I often see gems like this including the entire project structure for a dummy Rails application in their spec or test folder.

In the past there have been some gems such as plugin_test_helper and combustion that reduces the number of boilerplate files you need in your dummy application. However, I recently found that even without helper gems like these, you can define your entire Rails 3.x application in a single file.

Example Rails application

I based this example off this blog post and example by Julio Cesar Ody, and adapted it slightly to work in Rails 3.2.

Below is an example I use for testing middleware. I place this file in spec/dummy/application.rb.

require 'rails'
require 'action_controller/railtie'

class Dummy < Rails::Application
  # Set Rails.root to the same folder as this file
  config.root = File.dirname(__FILE__)

  # Rails needs these keys, but they don't really have to be secret for our tests
  config.session_store :cookie_store, key: '****************************************'
  config.secret_token = '****************************************'

  # Log to spec/dummy/test.log
  config.logger ='../test.log', __FILE__))
  # This is important, otherwise the tests will fail
  Rails.logger = config.logger

  # The middleware I'm testing
  config.middleware.use MyMiddleware

  # Our routes
  routes.draw do
    get '/'  => 'dummy#index'
    get '/other' => 'dummy#other'

# A simple controller
class DummyController < ActionController::Base
  def index
    render text: 'Home'

  def other
    render text: 'Other'

Using the application in tests/specs

For my tests I’m using rack/test. Using the application is as simple as requiring the application file, and telling rack/test to use the it:

require 'rspec'
require 'rack/test'

# Configure the Rails application
ENV['RAILS_ENV'] = 'test'
require 'dummy/application'

describe "rails specs" do
  # Tell rack/test to use our Rails dummy application
  let(:app) do

  it "should get the index page" do
    get '/'
    last_response.status.should == 200
    last_response.body.should =~ /Home/

That’s it! For a more complete example, check out my simple_admin_auth gem.

Published: May 03 2013

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