title: Django aggregate multiple columns after arithmetic operation
body: <p>I have a really strange problem with Django 1.4.4.</p>
  
  <p>I have this model :</p>
  
  <pre><code>class LogQuarter(models.Model):
    timestamp = models.DateTimeField()
    domain = models.CharField(max_length=253)
    attempts = models.IntegerField()
    success = models.IntegerField()
    queue = models.IntegerField()
    ...
  </code></pre>
  
  <p>I need to gather the first 20 domains with the higher sent property. The sent property is attempts - queue.</p>
  
  <p>This is my request:</p>
  
  <pre><code>obj = LogQuarter.objects\
        .aggregate(Sum(F('attempts')-F('queue')))\
        .values('domain')\
        .filter(**kwargs)\
        .order_by('-sent')[:20]
  </code></pre>
  
  <p>I tried with extra too and it isn't working.</p>
  
  <p>It's really basic SQL, I am surprised that Django can't do this.</p>
  
  <p>Did someone has a solution ?</p>