title: How to group notifications from different sources into one model
body: <p>What I have right now is a Roleplay Model, and a Message Model.</p>
  
  <p>I want them to share the same notification stream: They both should create a 'Notification' row which will then be displayed to the user. 
  But I want the Notification also point to their parent. I thought about creating a parent_id and parent_class column so that I could get the parent object. But is there an already built in way of doing it? </p>
  
  <p>I checked out Polymorphic Models, but I don't seem to really understand how to use it here.</p>