Fixed #30027 -- Errored out on Window function usage if unsupported.

django/db/models/expressions.py

@@ -7,6 +7,7 @@ from django.core.exceptions import EmptyResultSet, FieldError
 from django.db import connection
 from django.db.models import fields
 from django.db.models.query_utils import Q
+from django.db.utils import NotSupportedError
 from django.utils.deconstruct import deconstructible
 from django.utils.functional import cached_property
 from django.utils.hashable import make_hashable
@@ -1237,6 +1238,8 @@ class Window(Expression):
 
     def as_sql(self, compiler, connection, template=None):
         connection.ops.check_expression_support(self)
+        if not connection.features.supports_over_clause:
+            raise NotSupportedError('This backend does not support window expressions.')
         expr_sql, params = compiler.compile(self.source_expression)
         window_sql, window_params = [], []
 

tests/expressions_window/tests.py

@@ -1,5 +1,5 @@
 import datetime
-from unittest import skipIf, skipUnless
+from unittest import mock, skipIf, skipUnless
 
 from django.core.exceptions import FieldError
 from django.db import NotSupportedError, connection
@@ -821,6 +821,12 @@ class NonQueryWindowTests(SimpleTestCase):
         with self.assertRaisesMessage(NotSupportedError, msg):
             Employee.objects.annotate(dense_rank=Window(expression=DenseRank())).filter(dense_rank__gte=1)
 
+    def test_unsupported_backend(self):
+        msg = 'This backend does not support window expressions.'
+        with mock.patch.object(connection.features, 'supports_over_clause', False):
+            with self.assertRaisesMessage(NotSupportedError, msg):
+                Employee.objects.annotate(dense_rank=Window(expression=DenseRank())).get()
+
     def test_invalid_order_by(self):
         msg = 'order_by must be either an Expression or a sequence of expressions'
         with self.assertRaisesMessage(ValueError, msg):