title: Using .extra() on fields created by .annotate() in Django
body: <p>I want to retrieve a sum of two fields (which are aggregations themselves) for each object in a table.</p>
  
  <p>The following may describe a bit better what I'm after but results in an <code>Unknown column in field list</code>-Error: </p>
  
  <pre><code>items = MyModel.objects.annotate(
                  field1=Sum("relatedModel__someField"),
                  field2=Sum("relatedModel__someField")).extra(
                          select={"sum_field1_field2": "field1 + field2"})
  </code></pre>
  
  <p>I also tried using F() for the field lookups but that gives me an invalid sql statement.</p>
  
  <p>Any ideas on how to solve this are much appreciated.</p>