Refs #26780 -- Made prefetch_related() don't use window expressions fo sliced queries if not supported.

tests/prefetch_related/tests.py

@@ -2,11 +2,16 @@ from unittest import mock
 
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ObjectDoesNotExist
-from django.db import connection
+from django.db import NotSupportedError, connection
 from django.db.models import Prefetch, QuerySet, prefetch_related_objects
 from django.db.models.query import get_prefetcher
 from django.db.models.sql import Query
-from django.test import TestCase, override_settings
+from django.test import (
+    TestCase,
+    override_settings,
+    skipIfDBFeature,
+    skipUnlessDBFeature,
+)
 from django.test.utils import CaptureQueriesContext, ignore_warnings
 from django.utils.deprecation import RemovedInDjango50Warning
 
@@ -1911,6 +1916,7 @@ class NestedPrefetchTests(TestCase):
 
 
 class PrefetchLimitTests(TestDataMixin, TestCase):
+    @skipUnlessDBFeature("supports_over_clause")
     def test_m2m_forward(self):
         authors = Author.objects.all()  # Meta.ordering
         with self.assertNumQueries(3):
@@ -1924,6 +1930,7 @@ class PrefetchLimitTests(TestDataMixin, TestCase):
             with self.subTest(book=book):
                 self.assertEqual(book.authors_sliced, list(book.authors.all())[1:])
 
+    @skipUnlessDBFeature("supports_over_clause")
     def test_m2m_reverse(self):
         books = Book.objects.order_by("title")
         with self.assertNumQueries(3):
@@ -1937,6 +1944,7 @@ class PrefetchLimitTests(TestDataMixin, TestCase):
             with self.subTest(author=author):
                 self.assertEqual(author.books_sliced, list(author.books.all())[1:2])
 
+    @skipUnlessDBFeature("supports_over_clause")
     def test_foreignkey_reverse(self):
         authors = Author.objects.order_by("-name")
         with self.assertNumQueries(3):
@@ -1960,6 +1968,7 @@ class PrefetchLimitTests(TestDataMixin, TestCase):
                     list(book.first_time_authors.all())[1:],
                 )
 
+    @skipUnlessDBFeature("supports_over_clause")
     def test_reverse_ordering(self):
         authors = Author.objects.reverse()  # Reverse Meta.ordering
         with self.assertNumQueries(3):
@@ -1972,3 +1981,13 @@ class PrefetchLimitTests(TestDataMixin, TestCase):
         for book in books:
             with self.subTest(book=book):
                 self.assertEqual(book.authors_sliced, list(book.authors.all())[1:])
+
+    @skipIfDBFeature("supports_over_clause")
+    def test_window_not_supported(self):
+        authors = Author.objects.all()
+        msg = (
+            "Prefetching from a limited queryset is only supported on backends that "
+            "support window functions."
+        )
+        with self.assertRaisesMessage(NotSupportedError, msg):
+            list(Book.objects.prefetch_related(Prefetch("authors", authors[1:])))