title: Django: Annotate based on an annotation
body: <p>Let's say I'm using Django to manage a database about athletes:</p>
  
  <pre><code>class Player(models.Model):
      name = models.CharField()
      weight = models.DecimalField()
      team = models.ForeignKey('Team')
  
  class Team(models.Model):
      name = models.CharField()
      sport = models.ForeignKey('Sport')
  
  class Sport(models.Model):
      name = models.CharField()
  </code></pre>
  
  <p>Let's say I wanted to compute the average weight of the players on each team.  I think I'd do:</p>
  
  <pre><code>Team.objects.annotate(avg_weight=Avg(player__weight))
  </code></pre>
  
  <p>But now say that I want to compute the variance of team weights within each sport.  Is there a way to do that using the Django ORM?  How about using the <code>extra()</code> method on a QuerySet?  Any advice is much appreciated.</p>