A long-lived project that still receives updates
Helper for testing Rails Engines sanely


>= 0
>= 0


>= 5.0
 Project Readme

Gem Version


Rake tasks to generate a test application for a Rails Engine gem.

If you have a Rails Engine and want to test it, the suggested approach is a small dummy application that loads up a Rails application with your engine loaded. This dummy application is usually checked into source control and maintained with the application. This works great, until you want to test:

  • different versions of Ruby (e.g. MRI and JRuby)
  • different versions of Rails (Rails 6.1, 7.1)
  • different deployment environments (with and without devise, etc)

where each scenario may involve different configurations, Gemfiles, etc.

EngineCart helps by adding Rake tasks to your Engine that builds a disposable test application for you using Rails generators (and/or application templates).


Add this line to your engines's Gemfile:

gem 'engine_cart'

And then execute:

$ bundle

Or install it yourself as:

$ gem install engine_cart


engine_cart rake tasks

To use engine_cart's rake tasks, add the following to your engine's Rakefile (e.g. your_engine/Rakefile):

require 'engine_cart/rake_task'

Set up your engine to use engine_cart

Run engine_cart:prepare

In order for your Rails engine gem to use engine_cart, some configuration is required. There is an EngineCart generator to do this; it is also packaged as a rake task.

$ rake engine_cart:prepare

This will create ./spec/test_app_templates/lib/generators in your engine and it will also append some code to your engine's Gemfile.

You only need to run this rake task once.

Adjust your engine's generators

EngineCart is configured so it will run the test app generator (located in ./spec/test_app_templates/lib/generators) immediately after generating a testing rails app. By default, it will attempt to run the install generator for your engine. If you do not have an install generator, or want to add additional steps (e.g. to install additional gems), you can add them to the TestAppGenerator.

Generate the testing Rails application

You can generate a Rails testing application for your engine with a rake task:

$ rake engine_cart:generate

This creates a new Rails app containing your engine at .internal_test_app by running generators.

You can start the testing app, interact with it, etc:

$ bundle exec rake engine_cart:server

# or with arguments
$ bundle exec rake engine_cart:server["-p 3001 -e production"]

The testing app starts at http://localhost:3000, just like any Rails app.

If you need to perform additional debugging tasks, you can find the internal test application in the .internal_test_app directory. From there, you can do normal Rails things, like:

  • run rake tasks
  • run rails console

Running Tests using the Rails application

The easiest way to do this is via a rake task. In your engine's Rakefile:

require 'engine_cart/rake_task'

task :ci => ['engine_cart:generate'] do
  # run the tests

And in your engine's test framework configuration (e.g. spec_helper.rb or rails_helper.rb), initialize EngineCart:

require 'engine_cart'

Your test files (e.g. spec files for your engine) can now be written as tests for a Rails application.

Cleaning out or refreshing testing application

The EngineCart test application is meant to be disposable and easily rebuildable.

Automatic detection of when to rebuild test application

In some cases, EngineCart can automatically detect and rebuild the test application when key files change. By default, the application will be rebuilt when your Gemfile, Gemfile.lock, database migrations or generators change.

Rake tasks for rebuilding test application

You can also manually clean out and rebuild the test application. Run these rake tasks from the top level directory, not from the test application directory.

To clean out the testing app:

$ rake engine_cart:clean

Or, if you wish to start over immediately with a pristine testing application, the following will clean out the testing app and generate it anew:

$ rake engine_cart:regenerate

Brute force when all else fails

If you have generated a test application, there is a Gemfile and Gemfile.lock associated with the testing app at .internal_test_app (or wherever you designated as the test app location). If you then update your engine's Gemfile or .gemspec, Bundler can get confused if it has conflicting information between your engine and the testing app.

To fix this:

  1. Clean out your testing app: $ bundle exec rake engine_cart:clean or $ rm -rf .internal_test_app
  2. Remove your engine's Gemfile.lock: $ rm Gemfile.lock (not always necessary)
  3. Allow Bundler to resolve gem dependencies again: $ bundle install
  4. Rebuild the test application: $ bundle exec rake engine_cart:generate


Location of Rails testing app

You can configure where the test app is created by setting the ENGINE_CART_DESTINATION env variable, e.g.:

ENGINE_CART_DESTINATION="/tmp/my_engines_test_app_here" rake engine_cart:generate

Adjusting generators for Rails testing app

After creating the test application, EngineCart will run the test app generator (located in ./spec/test_app_templates/lib/generators). By default, it will attempt to run the install generator for your engine. If you do not have an install generator, or want to add additional steps (e.g. to install additional gems), you can add them to the TestAppGenerator.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a Pull Request