title: Django ordering parent model by count of children models in (one to many relation)
body: <p>Suppose we haw django models <code>Parent</code>, and <code>Child</code>. <code>Child</code> belongs to one <code>Parent</code> and one <code>Parent</code> can have multiple children.</p>
  
  <pre><code>class Parent(models.Model):
     pass
  
  class Child(models.Model):
    parent = models.ForeignKey(Parent)
  </code></pre>
  
  <p>I would like to obtain set of all parents ordered by the number of children.</p>