[1.8.x] Fixed #24418 -- Prevented crash in refresh_from_db with null fk

Thanks Johannes Lerch for the report, Tim Graham for the test case,
and Simon Charette for the review.
Backport of 5cf96b49e4 from master.

2 files changed, 8 insertions, 1 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index f75d20325f..4b51ca85d8 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -629,7 +629,7 @@ class Model(six.with_metaclass(ModelBase)):
             if field.rel and field.get_cache_name() in self.__dict__:
                 rel_instance = getattr(self, field.get_cache_name())
                 local_val = getattr(db_instance, field.attname)
-                related_val = getattr(rel_instance, field.related_field.attname)
+                related_val = None if rel_instance is None else getattr(rel_instance, field.related_field.attname)
                 if local_val != related_val:
                     del self.__dict__[field.get_cache_name()]
         self._state.db = db_instance._state.db
diff --git a/tests/basic/tests.py b/tests/basic/tests.py
index 53efba318d..ee6587ddce 100644
--- a/tests/basic/tests.py
+++ b/tests/basic/tests.py
@@ -736,6 +736,13 @@ class ModelRefreshTests(TestCase):
             self.assertFalse(hasattr(s3_copy.selfref, 'touched'))
             self.assertEqual(s3_copy.selfref, s2)
 
+    def test_refresh_null_fk(self):
+        s1 = SelfRef.objects.create()
+        s2 = SelfRef.objects.create(selfref=s1)
+        s2.selfref = None
+        s2.refresh_from_db()
+        self.assertEqual(s2.selfref, s1)
+
     def test_refresh_unsaved(self):
         pub_date = self._truncate_ms(datetime.now())
         a = Article.objects.create(pub_date=pub_date)