From 296b75a3c0309a936a6c07d8f711f722e3b96e63 Mon Sep 17 00:00:00 2001
From: Mariusz Felisiak <felisiak.mariusz@gmail.com>
Date: Fri, 6 Oct 2023 20:18:41 +0200
Subject: [PATCH] Fixed #34889 -- Fixed get_prefetch_queryset() fallback in
 prefetch_one_level().

Thanks Matt Westcott for the report.

Regression in cac94dd8aa2fb49cd2e06b5b37cf039257284bb0.
---
 django/db/models/query.py       |  7 +++---
 tests/prefetch_related/tests.py | 44 +++++++++++++++++++++++++++++++--
 2 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/django/db/models/query.py b/django/db/models/query.py
index 112530293332..de00bba8d75a 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -2550,6 +2550,9 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
             RemovedInDjango60Warning,
             stacklevel=2,
         )
+        queryset = None
+        if querysets := lookup.get_current_querysets(level):
+            queryset = querysets[0]
         (
             rel_qs,
             rel_obj_attr,
@@ -2557,9 +2560,7 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
             single,
             cache_name,
             is_descriptor,
-        ) = prefetcher.get_prefetch_queryset(
-            instances, lookup.get_current_querysets(level)
-        )
+        ) = prefetcher.get_prefetch_queryset(instances, queryset)
     # We have to handle the possibility that the QuerySet we just got back
     # contains some prefetch_related lookups. We don't want to trigger the
     # prefetch_related functionality by evaluating the query. Rather, we need
diff --git a/tests/prefetch_related/tests.py b/tests/prefetch_related/tests.py
index dd56664f6808..08f87d55de15 100644
--- a/tests/prefetch_related/tests.py
+++ b/tests/prefetch_related/tests.py
@@ -4,10 +4,12 @@
 from django.core.exceptions import ObjectDoesNotExist
 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.fields.related import ForwardManyToOneDescriptor
+from django.db.models.query import get_prefetcher, prefetch_one_level
 from django.db.models.sql import Query
 from django.test import (
     TestCase,
+    ignore_warnings,
     override_settings,
     skipIfDBFeature,
     skipUnlessDBFeature,
@@ -1997,7 +1999,7 @@ def test_window_not_supported(self):
             list(Book.objects.prefetch_related(Prefetch("authors", authors[1:])))
 
 
-class GetCurrentQuerySetDeprecation(TestCase):
+class DeprecationTests(TestCase):
     def test_get_current_queryset_warning(self):
         msg = (
             "Prefetch.get_current_queryset() is deprecated. Use "
@@ -2011,3 +2013,41 @@ def test_get_current_queryset_warning(self):
             )
         with self.assertWarnsMessage(RemovedInDjango60Warning, msg):
             self.assertIsNone(Prefetch("authors").get_current_queryset(1))
+
+    @ignore_warnings(category=RemovedInDjango60Warning)
+    def test_prefetch_one_level_fallback(self):
+        class NoGetPrefetchQuerySetsDescriptor(ForwardManyToOneDescriptor):
+            def get_prefetch_queryset(self, instances, queryset=None):
+                if queryset is None:
+                    return super().get_prefetch_querysets(instances)
+                return super().get_prefetch_querysets(instances, [queryset])
+
+            def __getattribute__(self, name):
+                if name == "get_prefetch_querysets":
+                    raise AttributeError
+                return super().__getattribute__(name)
+
+        house = House.objects.create()
+        room = Room.objects.create(house=house)
+        house.main_room = room
+        house.save()
+
+        # prefetch_one_level() fallbacks to get_prefetch_queryset().
+        prefetcher = NoGetPrefetchQuerySetsDescriptor(Room._meta.get_field("house"))
+        obj_list, additional_lookups = prefetch_one_level(
+            [room],
+            prefetcher,
+            Prefetch("house", House.objects.all()),
+            0,
+        )
+        self.assertEqual(obj_list, [house])
+        self.assertEqual(additional_lookups, [])
+
+        obj_list, additional_lookups = prefetch_one_level(
+            [room],
+            prefetcher,
+            Prefetch("house"),
+            0,
+        )
+        self.assertEqual(obj_list, [house])
+        self.assertEqual(additional_lookups, [])