title: django models to model 'has-a' and 'contains' relationships
body: <p>I have the following entities:</p>
  
  <pre><code>from django.db import models
  
  class Foo(models.Model):
      pass
  
  class Bar(models.Model):
      pass
  
  
  class FooBar(models.Model):
      pwned_foo = # Foobar contains one Foo object
      bars = # Collection of Bar objects
  </code></pre>
  
  <p>How do I express the relation between Foo, Bar and FooBar in the FooBar class?</p>