title: Extending the Spree::Product model/class
body: <p>I'm using Spree in a Rails 3.2 app of mine and I want to extend Spree's Product class to better suit my needs as for example to establish a relationship with another model in my app. What's the best way to do this? I could not find anything about it in the project documentation</p>
  
  <p>And what if I want to add new attributes/fields to the Product resource? I can't find it's migration either :/</p>
  
  <p>Thanks in advance :)</p>