[1.8.x] Fixed #25715 -- Fixed Model.refresh_from_db() with ForeignKey w/on_delete=SET_NULL.

Backport of 54e2e688e1 from master

django/db/models/base.py

@@ -630,7 +630,7 @@ class Model(six.with_metaclass(ModelBase)):
                 rel_instance = getattr(self, field.get_cache_name())
                 local_val = getattr(db_instance, field.attname)
                 related_val = None if rel_instance is None else getattr(rel_instance, field.related_field.attname)
-                if local_val != related_val:
+                if local_val != related_val or (local_val is None and related_val is None):
                     del self.__dict__[field.get_cache_name()]
         self._state.db = db_instance._state.db
 

docs/releases/1.8.7.txt

@@ -36,3 +36,6 @@ Bugfixes
   :class:`~django.contrib.postgres.fields.DateTimeRangeField` (:ticket:`24937`).
 
 * Fixed the exact lookup of ``ArrayField`` (:ticket:`25666`).
+
+* Fixed ``Model.refresh_from_db()`` updating of ``ForeignKey`` fields with
+  ``on_delete=models.SET_NULL`` (:ticket:`25715`).

tests/basic/models.py

@@ -30,6 +30,7 @@ class ArticleSelectOnSave(Article):
 class SelfRef(models.Model):
     selfref = models.ForeignKey('self', null=True, blank=True,
                                 related_name='+')
+    article = models.ForeignKey(Article, on_delete=models.SET_NULL, null=True, blank=True)
 
     def __str__(self):
         # This method intentionally doesn't work for all cases - part

tests/basic/tests.py

@@ -767,6 +767,17 @@ class ModelRefreshTests(TestCase):
         self.assertEqual(a2.pub_date, pub_date)
         self.assertEqual(a2._state.db, "default")
 
+    def test_refresh_fk_on_delete_set_null(self):
+        a = Article.objects.create(
+            headline='Parrot programs in Python',
+            pub_date=datetime(2005, 7, 28),
+        )
+        s1 = SelfRef.objects.create(article=a)
+        a.delete()
+        s1.refresh_from_db()
+        self.assertIsNone(s1.article_id)
+        self.assertIsNone(s1.article)
+
     def test_refresh_no_fields(self):
         a = Article.objects.create(pub_date=self._truncate_ms(datetime.now()))
         with self.assertNumQueries(0):