Autoloading The Wrong Superclass with Rails

May 2013 · about 2 minutes to read

Autoloading in Ruby on Rails can cause all kinds of grief if you don’t watch out. Here’s a really quick tip to avoid hours of debugging down the track.

Always use the full class namespace when specifying a superclass inside a module

For example:

module MyEngine
  class WidgetController < ApplicationController
    # BAD! ApplicationController is ambiguous
  end
end

module MyEngine
  class WidgetController < MyEngine::ApplicationController
    # Good - inherits from the module specific ApplicationController
  end
end

module MyEngine
  class WidgetController < ::ApplicationController
    # Good - inherits from the top-level ApplicationController
  end
end

In the first example, the superclass for WidgetController will depend on which classes are already loaded. Ruby will look for the following classes in order:

MyEngine::ApplicationController
::ApplicationController

The trouble occurs when Rails has already autoloaded ::ApplicationController and not MyEngine::ApplicationController. In this case, Rails will never need to autoload MyEngine::ApplicationController because ::ApplicationController already matches the superclass.

Autoloading can occur in a different order between execution environments. For example, tests could all pass and while production system fails because of the different autoloading behavior.

The examples below demonstrate how the order of the class definitions affect which superclass is used if the superclass is ambiguous. When written out like this, the behavior is fairly obvious.

class Base
  def initialize
    raise "This is the wrong super class!"
  end
end

module Widget
  class MyWidget < Base
    # Inherits from ::Base because Widget::Base isn't defined yet
  end
end

module Widget
  class Base
    def initialize
      puts "Success!"
    end
  end
end

> Widget::MyWidget.new
RuntimeError: This is the wrong super class!

class Base
  def initialize
    raise "This is the wrong super class!"
  end
end

module Widget
  class Base
    def initialize
      puts "Success!"
    end
  end
end

module Widget
  class MyWidget < Base
    # Inherits from Widget::Base
  end
end

> Widget::MyWidget.new
Success!