From 73df8b54a2fab53bec4c7573cda5ad8c869c2fd8 Mon Sep 17 00:00:00 2001
From: Giannis Terzopoulos <terzo.giannis@gmail.com>
Date: Fri, 9 Feb 2024 23:47:06 +0100
Subject: [PATCH] Fixed #35044 -- Avoided clearing reverse relations and
 private fields when accessing deferred fields.

Regression in a7b5ad8b19a08d7d57302ece74f6e26d2887fd9f for reverse
relations and possibly in 123b1d3fcf79f091573c40be6da7113a6ef35b62 for
private fields.
---
 django/db/models/base.py                |  8 ++++--
 tests/contenttypes_tests/test_fields.py | 35 +++++++++++++++++++++++++
 tests/defer/models.py                   |  8 ++++++
 tests/defer/tests.py                    | 26 ++++++++++++++++++
 4 files changed, 75 insertions(+), 2 deletions(-)

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 243093da44..3d40bc4286 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -740,12 +740,16 @@ class Model(AltersData, metaclass=ModelBase):
 
         # Clear cached relations.
         for field in self._meta.related_objects:
-            if field.is_cached(self):
+            if (fields is None or field.name in fields) and field.is_cached(self):
                 field.delete_cached_value(self)
 
         # Clear cached private relations.
         for field in self._meta.private_fields:
-            if field.is_relation and field.is_cached(self):
+            if (
+                (fields is None or field.name in fields)
+                and field.is_relation
+                and field.is_cached(self)
+            ):
                 field.delete_cached_value(self)
 
         self._state.db = db_instance._state.db
diff --git a/tests/contenttypes_tests/test_fields.py b/tests/contenttypes_tests/test_fields.py
index 5510f34cd0..15f1dafd63 100644
--- a/tests/contenttypes_tests/test_fields.py
+++ b/tests/contenttypes_tests/test_fields.py
@@ -45,6 +45,18 @@ class GenericForeignKeyTests(TestCase):
         new_entity = answer.question
         self.assertIsNot(old_entity, new_entity)
 
+    def test_clear_cached_generic_relation_explicit_fields(self):
+        question = Question.objects.create(text="question")
+        answer = Answer.objects.create(text="answer", question=question)
+        old_question_obj = answer.question
+        # The reverse relation is not refreshed if not passed explicitly in
+        # `fields`.
+        answer.refresh_from_db(fields=["text"])
+        self.assertIs(answer.question, old_question_obj)
+        answer.refresh_from_db(fields=["question"])
+        self.assertIsNot(answer.question, old_question_obj)
+        self.assertEqual(answer.question, old_question_obj)
+
 
 class GenericRelationTests(TestCase):
     def test_value_to_string(self):
@@ -55,6 +67,29 @@ class GenericRelationTests(TestCase):
         self.assertCountEqual(result, [answer1.pk, answer2.pk])
 
 
+class DeferredGenericRelationTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.question = Question.objects.create(text="question")
+        cls.answer = Answer.objects.create(text="answer", question=cls.question)
+
+    def test_defer_not_clear_cached_private_relations(self):
+        obj = Answer.objects.defer("text").get(pk=self.answer.pk)
+        with self.assertNumQueries(1):
+            obj.question
+        obj.text  # Accessing a deferred field.
+        with self.assertNumQueries(0):
+            obj.question
+
+    def test_only_not_clear_cached_private_relations(self):
+        obj = Answer.objects.only("content_type", "object_id").get(pk=self.answer.pk)
+        with self.assertNumQueries(1):
+            obj.question
+        obj.text  # Accessing a deferred field.
+        with self.assertNumQueries(0):
+            obj.question
+
+
 class GetPrefetchQuerySetDeprecation(TestCase):
     def test_generic_relation_warning(self):
         Question.objects.create(text="test")
diff --git a/tests/defer/models.py b/tests/defer/models.py
index 6de2e2af97..560e54c8c0 100644
--- a/tests/defer/models.py
+++ b/tests/defer/models.py
@@ -19,6 +19,14 @@ class Primary(models.Model):
         return self.name
 
 
+class PrimaryOneToOne(models.Model):
+    name = models.CharField(max_length=50)
+    value = models.CharField(max_length=50)
+    related = models.OneToOneField(
+        Secondary, models.CASCADE, related_name="primary_o2o"
+    )
+
+
 class Child(Primary):
     pass
 
diff --git a/tests/defer/tests.py b/tests/defer/tests.py
index c7eb03dc8a..3945b667ba 100644
--- a/tests/defer/tests.py
+++ b/tests/defer/tests.py
@@ -6,6 +6,7 @@ from .models import (
     Child,
     ChildProxy,
     Primary,
+    PrimaryOneToOne,
     RefreshPrimaryProxy,
     Secondary,
     ShadowChild,
@@ -326,3 +327,28 @@ class InvalidDeferTests(SimpleTestCase):
         )
         with self.assertRaisesMessage(FieldError, msg):
             Primary.objects.only("name").select_related("related")[0]
+
+
+class DeferredRelationTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.secondary = Secondary.objects.create(first="a", second="b")
+        cls.primary = PrimaryOneToOne.objects.create(
+            name="Bella", value="Baxter", related=cls.secondary
+        )
+
+    def test_defer_not_clear_cached_relations(self):
+        obj = Secondary.objects.defer("first").get(pk=self.secondary.pk)
+        with self.assertNumQueries(1):
+            obj.primary_o2o
+        obj.first  # Accessing a deferred field.
+        with self.assertNumQueries(0):
+            obj.primary_o2o
+
+    def test_only_not_clear_cached_relations(self):
+        obj = Secondary.objects.only("first").get(pk=self.secondary.pk)
+        with self.assertNumQueries(1):
+            obj.primary_o2o
+        obj.second  # Accessing a deferred field.
+        with self.assertNumQueries(0):
+            obj.primary_o2o