[5.1.x] Fixed #35950 -- Restored refreshing of relations when fields deferred.

Thank you to Simon Charette and Sarah Boyce for the review.

Regression in 73df8b54a2fab53bec4c7573cda5ad8c869c2fd8.

Backport of 2f6b096b83c55317c7ceef2d8d5dc3bee33293dc from main.

django/db/models/base.py

@@ -717,12 +717,13 @@ class Model(AltersData, metaclass=ModelBase):
         if fields is not None:
             db_instance_qs = db_instance_qs.only(*fields)
         elif deferred_fields:
-            fields = {
-                f.attname
-                for f in self._meta.concrete_fields
-                if f.attname not in deferred_fields
-            }
-            db_instance_qs = db_instance_qs.only(*fields)
+            db_instance_qs = db_instance_qs.only(
+                *{
+                    f.attname
+                    for f in self._meta.concrete_fields
+                    if f.attname not in deferred_fields
+                }
+            )
 
         db_instance = db_instance_qs.get()
         non_loaded_fields = db_instance.get_deferred_fields()
@@ -739,9 +740,9 @@ class Model(AltersData, metaclass=ModelBase):
                     field.delete_cached_value(self)
 
         # Clear cached relations.
-        for field in self._meta.related_objects:
-            if (fields is None or field.name in fields) and field.is_cached(self):
-                field.delete_cached_value(self)
+        for rel in self._meta.related_objects:
+            if (fields is None or rel.name in fields) and rel.is_cached(self):
+                rel.delete_cached_value(self)
 
         # Clear cached private relations.
         for field in self._meta.private_fields:

docs/releases/5.1.4.txt

@@ -12,3 +12,7 @@ Bugfixes
 
 * Fixed a crash in ``createsuperuser`` on Python 3.13+ caused by an unhandled
   ``OSError`` when the username could not be determined (:ticket:`35942`).
+
+* Fixed a regression in Django 5.1 where relational fields were not updated
+  when calling ``Model.refresh_from_db()`` on instances with deferred fields
+  (:ticket:`35950`).

tests/contenttypes_tests/test_fields.py

@@ -57,6 +57,15 @@ class GenericForeignKeyTests(TestCase):
         self.assertIsNot(answer.question, old_question_obj)
         self.assertEqual(answer.question, old_question_obj)
 
+    def test_clear_cached_generic_relation_when_deferred(self):
+        question = Question.objects.create(text="question")
+        Answer.objects.create(text="answer", question=question)
+        answer = Answer.objects.defer("text").get()
+        old_question_obj = answer.question
+        # The reverse relation is refreshed even when the text field is deferred.
+        answer.refresh_from_db()
+        self.assertIsNot(answer.question, old_question_obj)
+
 
 class GenericRelationTests(TestCase):
     def test_value_to_string(self):

tests/defer/tests.py

@@ -290,6 +290,14 @@ class TestDefer2(AssertionMixin, TestCase):
             self.assertEqual(rf2.name, "new foo")
             self.assertEqual(rf2.value, "new bar")
 
+    def test_refresh_when_one_field_deferred(self):
+        s = Secondary.objects.create()
+        PrimaryOneToOne.objects.create(name="foo", value="bar", related=s)
+        s = Secondary.objects.defer("first").get()
+        p_before = s.primary_o2o
+        s.refresh_from_db()
+        self.assertIsNot(s.primary_o2o, p_before)
+
 
 class InvalidDeferTests(SimpleTestCase):
     def test_invalid_defer(self):