Aggregate Sum of GIS Aggregates fails

[Daniel Wiesmann]

There is an error when trying to compute a regular aggregate value from a GIS Area aggregate output. To take an example from the GIS geoapp tests, the following query fails.

from django.db.models import Sum
from django.contrib.gis.db.models.functions import Area

Country.objects.annotate(areasum=Sum(Area('mpoly')))

I tracked this down to the from_db_value function in the GeometryField class here:

​https://github.com/django/django/blob/master/django/contrib/gis/db/models/fields.py#L259

The problem seems to be that the Area function returns a float, but the field is expecting a value that can be converted to a Geometry.

I'll open a pull request with a regression test and a proposed solution.