Fix update_change_reason: fails to fetch latest history entry for nullable fields

AUTHORS.rst

@@ -147,6 +147,7 @@ Authors
 - `Sridhar Marella <https://github.com/sridhar562345>`_
 - `Mattia Fantoni <https://github.com/MattFanto>`_
 - `Trent Holliday <https://github.com/trumpet2012>`_
+- Rahul Mishra (`Senor-Ducky <https://github.com/Senor-Ducky>`_)
 
 Background
 ==========

simple_history/tests/tests/test_utils.py

@@ -640,3 +640,13 @@ def test_update_change_reason_with_excluded_fields(self):
         update_change_reason(poll, "Test change reason.")
         most_recent = poll.history.order_by("-history_date").first()
         self.assertEqual(most_recent.history_change_reason, "Test change reason.")
+
+    def test_update_change_reason_with_null_fields(self):
+        poll = PollWithExcludeFields(
+            question="what's up?", pub_date=timezone.now(), place=None
+        )
+        poll.save()
+        update_change_reason(poll, "Test change reason.")
+        most_recent = poll.history.order_by("-history_date").first()
+        self.assertIsNone(most_recent.place)
+        self.assertEqual(most_recent.history_change_reason, "Test change reason.")

simple_history/utils.py

@@ -19,7 +19,10 @@ def update_change_reason(instance, reason):
             if value is not None:
                 attrs[field.attname] = value
         else:
-            attrs[field.attname] = value
+            if value is not None:
+                attrs[field.attname] = value
+            else:
+                attrs[f"{field.attname}__isnull"] = True
 
     record = history.filter(**attrs).order_by("-history_date").first()
     record.history_change_reason = reason