We’ve been prepending classes a lot recently as a means of mokeypatching, but in a “nicer” way. Turns out you can do this with modules too.

Let’s say you’ve got a class in a gem somewhere and you want to override some of its behaviors.

    module Roaring
      def roar
        "Roar!"
      end
    end

    class MythicalBeast
      include Roaring
    end

The default looks like:

    class Dragon < MythicalBeast
    end

    >  Dragon.new.roar
    => "Roar!"

You can change that roar using prepend. Typically, this involves changing the class:

    module BetterRoar
      def roar
        "RRRROOOOOAAAARRRRRRRRR!!!!!!!"
      end
    end

    >  MythicalBeast.prepend BetterRoar
    >  Dragon.new.roar
    => "RRRROOOOOAAAARRRRRRRRR!!!!!!!"

But, you can can also do this on the module:

    >  Roaring.prepend BetterRoar
    >  Dragon.new.roar
    => "RRRROOOOOAAAARRRRRRRRR!!!!!!!"

The only different is that you’ll need to reload the classes, which Rails will do in the to_prepare of the application config.

Why do this?

Well, it’s really up to you, but prepending the module instead of the class is a bit more self-documenting because in your config.to_prepare block you’d see something like:

    config.to_preprae do
      Roaring.prepend BetterRoar
    end

This is a bit more explicit in the sense that it’s telling you where the original behavior exists that you’re changing. If you chose to prepend the class:

    config.to_preprae do
      MythicalBeast.prepend BetterRoar
    end

You don’t know that the roar method really existing in the Roaring module.