Fixed #18247 -- Added cast to NUMERIC for Decimals on sqlite

On sqlite the SUM() of a decimal column doesn't have a NUMERIC type so
when comparing it to a string literal (which a Decimal gets converted to
in Django) it is not compared as expected.

django/db/models/expressions.py

@@ -507,6 +507,15 @@ class Func(Expression):
         template = template or self.extra.get('template', self.template)
         return template % self.extra, params
 
+    def as_sqlite(self, *args, **kwargs):
+        sql, params = self.as_sql(*args, **kwargs)
+        try:
+            if self.output_field.get_internal_type() == 'DecimalField':
+                sql = 'CAST(%s AS NUMERIC)' % sql
+        except FieldError:
+            pass
+        return sql, params
+
     def copy(self):
         copy = super(Func, self).copy()
         copy.source_expressions = self.source_expressions[:]

tests/aggregation_regress/tests.py

@@ -349,6 +349,20 @@ class AggregationTests(TestCase):
             {'c__max': 3}
         )
 
+    def test_decimal_aggregate_annotation_filter(self):
+        """
+        Filtering on an aggregate annotation with Decimal values should work.
+        Requires special handling on SQLite (#18247).
+        """
+        self.assertEqual(
+            len(Author.objects.annotate(sum=Sum('book_contact_set__price')).filter(sum__gt=Decimal(40))),
+            1
+        )
+        self.assertEqual(
+            len(Author.objects.annotate(sum=Sum('book_contact_set__price')).filter(sum__lte=Decimal(40))),
+            4
+        )
+
     def test_field_error(self):
         # Bad field requests in aggregates are caught and reported
         self.assertRaises(