title: Django: Sum the count of a sub sub foreign object
body: <p>I have many hits by Twavail
  I have many Twavail for a Twaffic</p>
  
  <p>I would like to sum the number of Hits for a Twaffic, and get an additionel field in Twaffic entries (hits_count), but I have some difficulties.</p>
  
  <hr>
  
  <p>Here are my models:</p>
  
  <pre><code>class Twaffic(models.Model):
      user = models.ForeignKey(User,editable=False)
      enchere = models.PositiveIntegerField(default=1)
  
  class Twavail(models.Model):
  
      user = models.ForeignKey(User,editable=False)
      twaffic = models.ForeignKey(Twaffic,editable=False,related_name='twavaux')
      tweet_id = models.CharField(max_length=20,default=0, editable=False)
  
  class Hit(models.Model):
      twavail = models.ForeignKey(Twavail,editable=False,related_name='hits')
      date = models.DateTimeField(max_length=140,auto_now=True,default="")
  </code></pre>
  
  <p>I tried:</p>
  
  <pre><code>twaffics=Twaffic.objects.annotate(hits_count=Sum('twavaux__hits__count')).filter(user=request.user)
  </code></pre>
  
  <p>But I get the following error:</p>
  
  <p>Cannot resolve keyword 'hits' into field. Choices are: twaffic, user</p>
  
  <p>Is it really impossible to get this informations 2 levels ago? hits is the right related name.. Maybe I missed something?</p>
  
  <p>Thx for your help.</p>