title: Django: Sum on an date attribute grouped by month/year
body: <p>I'd like to put this query from SQL to Django:</p>
  
  <pre><code>"select date_format(date, '%Y-%m') as month, sum(quantity) as hours from hourentries group by date_format(date, '%Y-%m') order by date;"
  </code></pre>
  
  <p>The part that causes problem is to group by month when aggregating. I tried this (which seemed logical), but it didn't work :</p>
  
  <pre><code>HourEntries.objects.order_by("date").values("date__month").aggregate(Sum("quantity"))
  </code></pre>