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Wagons are extensions to your application train running on Rails
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Here Be Wagons

Wagons are extensions to your application train running on Rails. You can see them as plugins that extend the behavior of your specific Rails application. This framework makes it easy to create and manage them.

First of all, wagons are basically Rails Engines, so make sure you are familiar with them. Wagons provide a handful of additions so your wagon engines actually know your application.

Wagons differ from engines in a few points:

  • Wagons extend your application, engines extend Rails.

  • Wagon migrations are kept separately from your application's migrations to enable easy addition and removal of wagons.

  • When developing and testing, wagons use the main application instead of a dummy application.


As always, add this declaration to your application's Gemfile:

gem 'wagons'

Now you are ready for your first wagon. Generate it with

rails generate wagon [name]

This creates the structure of your wagon in vendor/wagons/[name]. In there, you find the file lib/[name]/wagon.rb, which defines the Rails::Engine and includes the Wagon module. Here, you may also extend your application classes in a config.to_prepare block.

In order to load wagons with the application, an entry in the Gemfile would be sufficient. To keep things flexible, wagons come with an additional file Wagonfile. Generate one for development purposes with:

rake wagon:file

This will include all wagons found in vendor/wagons in development mode. Do not check Wagonfile into source control. In your deployments, you might want to have different entries in there.

Once your wagon is ready to ship, a gem can be built with rake build. The name of a wagon gem must always start with the application name, followed with an underscore and the actual name of the wagon. In production, you may simply install the wagon gem and explicitly add a declaration to your Wagonfile.

If your wagon contains migrations and probably seed data, update your database with

rake wagon:setup WAGON=[name]

Leave off the WAGON parameter to setup all wagons in your Wagonfile. This should not interfere with wagons that are already set up. Migrations are only run if they are not loaded yet, as usual.

Extending your application with a wagon

Ruby and Rails provide all the magic required to extend your application from within a wagon.

To add new models, controllers or views, simply create them in the app directory of your wagon, as you would in a regular engine.

To extend existing models and controllers, you may create modules with the required functionality. Include them into your application classes in a config.to_prepare block in lib/[wagon_name]/wagon.rb.

To extend views, wagons provides a simple view helper that looks for partials in all view paths. Any template that might be extended by a wagon can include a call like this:

<%= render_extensions :details %>

Any partials living in an equally named subfolder as the calling template and starting with the given key are rendered at this place.

Wagon dependencies

Wagons may depend on each other and/or have certain requirements on their load order. To make sure something is loaded before the current wagon, add a require '[app_name]_[other_wagon]' on top of the lib/[app_name]_[current_wagon].rb file. For development dependencies, there is an extra require_optional method that will not raise a LoadError if the dependency is not found.

To control that the main application actually supports a certain wagon, an application version may be defined so wagons can define a requirement. The application version can be set in an initializer. Create it with:

rake wagon:app_version

Besides setting the version, this initializer will check all loaded wagons for their application requirement and raise an errors if one is not met. In lib/[wagon_name]/wagon.rb the requirement may be defined, e.g.:

app_requirement '>= 1.0'

The syntax follows the Ruby gem version and requirements.

Seed Data

Wagons integrates Seed Fu for seed data. All seed data from the application is also available in wagon tests, as long as no fixture files overwrite the corresponding tables.

Wagons may come with their own seed data as well. Simply put it into db/fixtures[/environment]. To allow for an automatic removal of wagons, Seed Fu-ndo is able to record seed file instructions and destroy all entries that exist in the database. Just make sure that you only use the seed and seed_once methods in these files, or the unseed may not work correctly.


There are a few other things that work differently with wagons:

Schema & Migrations

Wagons are extensions to your application that may vary between various installations. Wagon tables are added and removed as wagons are installed or uninstalled. After you have added a wagon's gem to your production Wagonfile, run rake wagon:setup to run the migrations and load the seed data. Before you remove them from Wagonfile, run rake wagon:remove WAGON=to_remove to eliminate the artifacts from the database first.

In this way, the schema.rb file must only contain the tables of the application, not of all wagons. When you have migrations for your main application and wagons loaded, the schema will not be dumped on db:migrate. You need to either remove the wagons or reset the database before the schema may be dumped. This is (currently) the cost for having arbitrary pluggable application extensions.


Wagons use your application for tests. This is also true for your application's test database. To get the correct setup, app:db:test:prepare is extended to run the migration of the current wagon and all its dependencies, as well as their seed data. Once the database is set up, single tests may be run with the usual ruby -I test test/my_test.rb command.

The test_helper.rb of the main application is included in all wagon tests. Any additions in this file are available in wagon tests as well. The only thing wagons need to do is reseting the fixture path to the wagon's test fixtures.

RSpec works fine with wagons as well. Simply put the heading lines found in test_helper.rb into your spec_helper.rb.

Gem Bundling

Bundler manages application dependencies, with a stress on application. Because wagons live inside your application during development, the app's Gemfile is included in each wagon's Gemfile. However, Bundler still keeps an own Gemfile.lock for each wagon, so you have to make sure to keep these up to date when you change your main application gems. The gem versions for the wagons should be the same as for the application. rake wagon:bundle:update is here to help you exactly with that. We recommend to NOT check in the Wagon's Gemfile.lock file into source control.

Unfortunately, adding wagon gems to the Wagonfile in production also breaks with Bundler's approach of locking down application gems. Because of that, the --deployment option cannot be used with wagons. If you install your gems from vendor/cache into vendor/bundle or so, you still get most of the benefits of using Bundler, including the guarantee for the very same gem versions as used in development.

Contributions to this or any other issues are very welcome.

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