title: Filter model on reverse_set length
body: <p>Consider these pseudo classes:</p>
  
  <pre><code>class Foo(models.Model):
      pass
  
  class Bar(models.Model):
      foo = models.ForeignKey(Foo)
  </code></pre>
  
  <p>I would like to filter via Foo's manager effectively to get a QuerySet that only holds Foo objects with atleast 2 Bar objects pointing towards it.</p>