Fix BaseQuerySet#sum and BaseQuerySet#average for fields that specify a db_field

mongoengine/queryset/base.py

@@ -1271,9 +1271,10 @@ def sum(self, field):
         :param field: the field to sum over; use dot notation to refer to
             embedded document fields
         """
+        db_field = self._fields_to_dbfields([field]).pop()
         pipeline = [
             {'$match': self._query},
-            {'$group': {'_id': 'sum', 'total': {'$sum': '$' + field}}}
+            {'$group': {'_id': 'sum', 'total': {'$sum': '$' + db_field}}}
         ]
 
         # if we're performing a sum over a list field, we sum up all the
@@ -1300,9 +1301,10 @@ def average(self, field):
         :param field: the field to average over; use dot notation to refer to
             embedded document fields
         """
+        db_field = self._fields_to_dbfields([field]).pop()
         pipeline = [
             {'$match': self._query},
-            {'$group': {'_id': 'avg', 'total': {'$avg': '$' + field}}}
+            {'$group': {'_id': 'avg', 'total': {'$avg': '$' + db_field}}}
         ]
 
         # if we're performing an average over a list field, we average out

tests/queryset/queryset.py

@@ -2838,6 +2838,34 @@ def test_sum(self):
             sum([a for a in ages if a >= 50])
         )
 
+    def test_sum_over_db_field(self):
+        """Ensure that a field mapped to a db field with a different name
+        can be summed over correctly.
+        """
+        class UserVisit(Document):
+            num_visits = IntField(db_field='visits')
+
+        UserVisit.drop_collection()
+
+        UserVisit.objects.create(num_visits=10)
+        UserVisit.objects.create(num_visits=5)
+
+        self.assertEqual(UserVisit.objects.sum('num_visits'), 15)
+
+    def test_average_over_db_field(self):
+        """Ensure that a field mapped to a db field with a different name
+        can have its average computed correctly.
+        """
+        class UserVisit(Document):
+            num_visits = IntField(db_field='visits')
+
+        UserVisit.drop_collection()
+
+        UserVisit.objects.create(num_visits=20)
+        UserVisit.objects.create(num_visits=10)
+
+        self.assertEqual(UserVisit.objects.average('num_visits'), 15)
+
     def test_embedded_average(self):
         class Pay(EmbeddedDocument):
             value = DecimalField()