Refs #28147 -- Fixed setting of OneToOne and Foreign Key fields to None when using attnames.

Regression in 519016e5f2.

django/db/models/fields/related.py

@@ -19,9 +19,9 @@ from django.utils.translation import gettext_lazy as _
 from . import Field
 from .mixins import FieldCacheMixin
 from .related_descriptors import (
-    ForwardManyToOneDescriptor, ForwardOneToOneDescriptor,
-    ManyToManyDescriptor, ReverseManyToOneDescriptor,
-    ReverseOneToOneDescriptor,
+    ForeignKeyDeferredAttribute, ForwardManyToOneDescriptor,
+    ForwardOneToOneDescriptor, ManyToManyDescriptor,
+    ReverseManyToOneDescriptor, ReverseOneToOneDescriptor,
 )
 from .related_lookups import (
     RelatedExact, RelatedGreaterThan, RelatedGreaterThanOrEqual, RelatedIn,
@@ -764,7 +764,7 @@ class ForeignKey(ForeignObject):
     By default ForeignKey will target the pk of the remote model but this
     behavior can be changed by using the ``to_field`` argument.
     """
-
+    descriptor_class = ForeignKeyDeferredAttribute
     # Field flags
     many_to_many = False
     many_to_one = True

django/db/models/fields/related_descriptors.py

@@ -67,9 +67,17 @@ from django.core.exceptions import FieldError
 from django.db import connections, router, transaction
 from django.db.models import Q, signals
 from django.db.models.query import QuerySet
+from django.db.models.query_utils import DeferredAttribute
 from django.utils.functional import cached_property
 
 
+class ForeignKeyDeferredAttribute(DeferredAttribute):
+    def __set__(self, instance, value):
+        if instance.__dict__.get(self.field.attname) != value and self.field.is_cached(instance):
+            self.field.delete_cached_value(instance)
+        instance.__dict__[self.field.attname] = value
+
+
 class ForwardManyToOneDescriptor:
     """
     Accessor to the related object on the forward side of a many-to-one or