title: Generic associations in SQLAlchemy with multiple parents
body: <p>I'm trying to get my head around the "<a href="http://techspot.zzzeek.org/files/2007/discriminator_on_association.py" rel="nofollow">Discriminator on association</a>" example shipped with SQLAlchemy, which defines HasAddresses mixin so each model subclassing HasAddresses magically gets an <code>addresses</code> attribute, which is a collection to which Address objects can be added. The linking is performed through an intermediate table so at the first glance the relationship looks like many-to-many, I hoped to be able to have multiple Addresses linked to a Customer, AND also multiple Customers and Suppliers linked to an Address.</p>
  
  <p>The <code>Address</code> model, however, is set up in such a way that it has a single <code>parent</code> attribute which can only reference a single object. So, in the example, an Address can only be linked to a single Customer or Supplier.</p>
  
  <p>How do I modify that example so Address is able to back-reference multiple parent objects? </p>