Fixed #26112 -- Error when computing aggregate of GIS areas.

Thanks Simon Charette and Claude Paroz for the reviews.

tests/gis_tests/geoapp/models.py

@@ -22,6 +22,10 @@ class Country(NamedModel):
     mpoly = models.MultiPolygonField()  # SRID, by default, is 4326
 
 
+class CountryWebMercator(NamedModel):
+    mpoly = models.MultiPolygonField(srid=3857)
+
+
 class City(NamedModel):
     point = models.PointField()
 

tests/gis_tests/geoapp/test_functions.py

@@ -5,12 +5,14 @@ from decimal import Decimal
 
 from django.contrib.gis.db.models import functions
 from django.contrib.gis.geos import LineString, Point, Polygon, fromstr
+from django.contrib.gis.measure import Area
 from django.db import connection
+from django.db.models import Sum
 from django.test import TestCase, skipUnlessDBFeature
 from django.utils import six
 
 from ..utils import mysql, oracle, postgis, spatialite
-from .models import City, Country, State, Track
+from .models import City, Country, CountryWebMercator, State, Track
 
 
 @skipUnlessDBFeature("gis_enabled")
@@ -231,6 +233,20 @@ class GISFunctionsTests(TestCase):
                 expected = c.mpoly.intersection(geom)
             self.assertEqual(c.inter, expected)
 
+    @skipUnlessDBFeature("has_Area_function")
+    def test_area_with_regular_aggregate(self):
+        # Create projected country objects, for this test to work on all backends.
+        for c in Country.objects.all():
+            CountryWebMercator.objects.create(name=c.name, mpoly=c.mpoly)
+        # Test in projected coordinate system
+        qs = CountryWebMercator.objects.annotate(area_sum=Sum(functions.Area('mpoly')))
+        for c in qs:
+            result = c.area_sum
+            # If the result is a measure object, get value.
+            if isinstance(result, Area):
+                result = result.sq_m
+            self.assertAlmostEqual((result - c.mpoly.area) / c.mpoly.area, 0)
+
     @skipUnlessDBFeature("has_MemSize_function")
     def test_memsize(self):
         ptown = City.objects.annotate(size=functions.MemSize('point')).get(name='Pueblo')