title: how to group_by a query by date and sum 2 others fields
body: <p>My model looks like this :</p>
  
  <pre><code>class PostMetric(models.Model):
      likes = models.IntegerField()
      comments = models.IntegerField()
      created_at = models.DateTimeField(auto_now_add=True)
  </code></pre>
  
  <p>I would like to get the sum of likes and comments for each day (note that i need to transform the datetimefield to datefield (probably using Cast?):</p>
  
  <pre><code>   date       engagement (likes+comments)
   2/5/2017         26
   3/5/2017         29
  </code></pre>
  
  <p>I tried to use : Cast, annotate, aggregate and even rawsql and extra/select functions but with no success.</p>
  
  <p>I think that i need to add an aggregation based to this query :</p>
  
  <pre><code>PostMetric.annotate(engagement=Sum('likes') + Sum('comments')).annotate(date_only=Cast('created_at', DateField()))
  </code></pre>