Using .extra() on fields created by .annotate() in Django

I want to retrieve a sum of two fields (which are aggregations themselves) for each object in a table.

The following may describe a bit better what I'm after but results in an Unknown column in field l开发者_如何学Pythonist-Error:

items = MyModel.objects.annotate(
                field1=Sum("relatedModel__someField"),
                field2=Sum("relatedModel__someField")).extra(
                        select={"sum_field1_field2": "field1 + field2"})

I also tried using F() for the field lookups but that gives me an invalid sql statement.

Any ideas on how to solve this are much appreciated.

it this what you want?

items = MyModel.objects.extra(
    select = {'sum_field1_field2': 'SUM(relatedModel__someField) + SUM(relatedModel__someField)'},
)

To make it work for many to many or for many to one (reverse) relations, you may use the following:

items = MyModel.objects.extra(
    select = {'sum_field1_field2': 'SUM("relatedModel"."someField") + SUM("relatedModel"."someField")'},
      )

But this will break also if you need another annotate, like for a count, because extra will add the statement to the GROUP BY clause, whereas aggregate functions are not allowed in there.