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

Thanks Simon Charette and Claude Paroz for the reviews.

django/contrib/gis/db/models/functions.py

@@ -117,24 +117,23 @@ class OracleToleranceMixin(object):
 
 
 class Area(OracleToleranceMixin, GeoFunc):
+    output_field_class = AreaField
     arity = 1
 
     def as_sql(self, compiler, connection):
         if connection.ops.geography:
-            # Geography fields support area calculation, returns square meters.
-            self.output_field = AreaField('sq_m')
-        elif not self.output_field.geodetic(connection):
-            # Getting the area units of the geographic field.
-            units = self.output_field.units_name(connection)
-            if units:
-                self.output_field = AreaField(
-                    AreaMeasure.unit_attname(self.output_field.units_name(connection))
-                )
-            else:
-                self.output_field = FloatField()
+            self.output_field.area_att = 'sq_m'
         else:
+            # Getting the area units of the geographic field.
+            source_fields = self.get_source_fields()
+            if len(source_fields):
+                source_field = source_fields[0]
+                if source_field.geodetic(connection):
                     # TODO: Do we want to support raw number areas for geodetic fields?
                     raise NotImplementedError('Area on geodetic coordinate systems not supported.')
+                units_name = source_field.units_name(connection)
+                if units_name:
+                    self.output_field.area_att = AreaMeasure.unit_attname(units_name)
         return super(Area, self).as_sql(compiler, connection)
 
     def as_oracle(self, compiler, connection):

django/contrib/gis/db/models/sql/conversion.py

@@ -21,13 +21,14 @@ class BaseField(object):
 
 class AreaField(BaseField):
     "Wrapper for Area values."
-    def __init__(self, area_att):
+    def __init__(self, area_att=None):
         self.area_att = area_att
 
     def from_db_value(self, value, expression, connection, context):
         if connection.features.interprets_empty_strings_as_nulls and value == '':
             value = None
-        if value is not None:
+        # If the units are known, convert value into area measure.
+        if value is not None and self.area_att:
             value = Area(**{self.area_att: value})
         return value
 

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')