Extending Rails Engins with the Decorators Ruby Gem

Over the past month I have been launching a few sites using a neat little Rails engine called Piggybak. Piggybak (worth a blog post on its own) handles just the basics of an e-store, and leaves the rest up for the creative developer.

While building these sites I would run into scenarios where I needed to extend the engine’s functionality and I went searching for an elegant “railsy” way of doing so. My search led me to some threads from the Forem project, and ultimately to the Decorators gem.


I couldn’t readily find any how-to’s on the gem so I dug into the code, and the usage in the Forem gem. It is super simple to use, but I figured I would add some documentation here on the blog. First things first, to utilize Decorators in your rails project add it to your Gemfile

gem 'decorators'

The gem does have some opinions on how one should construct their decorators for Rails engines. I based my usage upon what was found in Forem and created a decorators folder in the app root of my Rails.root. Within that folder, one should then create folders that mimic your extensions/decorations to engines (i.e. /app/decorators/models, /app/decorators/views etc), and the decorators themselves need to have a suffix of _decorator.rb.


So lets decorate the Piggybak::Order model to include an association for packages. Following the folder structure above, we would create a file at /app/decorators/models/piggybak/order_decorator.rb like so:


The last piece to take care of is add the following line to a config/initializer file:

Decorators.register! Rails.root