Testing Rails Engine Gems

I’ve been working on a number of gems that are basically packaged Ruby on Rails engine plugins. It turns out that turning gems into plugins is pretty easy to do. However, testing them can be a pain. Here are a few things I came up with.

After looking at how Clearance handles tests I’ve decided that it’s ok to embed a basic Rails application in your test directory.

The next trick is getting the gem you are working on to load in your embedded Rails application. The Clearance guys added a file in config/initializers that looks like this:


# This simulates loading the clearance gem, but without relying on
# vendor/gems

clearance_path = File.join(File.dirname(__FILE__), *%w(.. .. .. ..))
clearance_lib_path = File.join(clearance_path, "lib")

$LOAD_PATH.unshift(clearance_lib_path)
load File.join(clearance_path, 'rails', 'init.rb')
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I thought that was brilliant but it didn't work for me.  After messing around in the Rails code for a bit I found a bit of helpful documentation on 'plugin_locators':


The classes that handle finding the desired plugins that youâ€˜d like to load for your application. By default it is the Rails::Plugin::FileSystemLocator which finds plugins to load in vendor/plugins. You can hook into gem location by subclassing Rails::Plugin::Locator and adding it onto the list of plugin_locators.


That sounded like the perfect solution to my problem.  I figured I would simply add a plugin locator that pointed to the root of the gem I was building.  Turns out that works great.  In the embedded rails application inside of config/environment.rb I added this code:


class TestGemLocator < Rails::Plugin::Locator
  def plugins
    Rails::Plugin.new(File.join(File.dirname(__FILE__), *%w(.. .. ..)))
  end
end

Rails::Initializer.run do |config|
  config.time_zone = 'UTC'
  config.plugin_locators << TestGemLocator
end
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Now my tests load correctly and all is well.  Context is important.  If you want to have a look at the full project I put it into a gem called disguise which you can get from github.

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