Fix None type mismatch in change detection (#763)

* Add test model and test cases for None value type mismatch

- issue #750 @The-Alchemist

* Fix None type mismatch in change detection

auditlog/diff.py

@@ -73,15 +73,15 @@ def get_field_value(obj, field, use_json_for_changes=False):
         try:
             model_field = obj._meta.get_field(field.name)
             default = model_field.default
-            if default is NOT_PROVIDED:
-                return None
-
-            if callable(default):
-                return default()
-
-            return default
         except AttributeError:
-            return None
+            default = NOT_PROVIDED
+
+        if default is NOT_PROVIDED:
+            default = None
+        elif callable(default):
+            default = default()
+
+        return smart_str(default) if not use_json_for_changes else default
 
     try:
         if isinstance(field, DateTimeField):

auditlog_tests/test_app/models.py

@@ -442,6 +442,13 @@ class CustomMaskModel(models.Model):
     history = AuditlogHistoryField(delete_related=True)
 
 
+class NullableFieldModel(models.Model):
+    time = models.TimeField(null=True, blank=True)
+    optional_text = models.CharField(max_length=100, null=True, blank=True)
+
+    history = AuditlogHistoryField(delete_related=True)
+
+
 auditlog.register(AltPrimaryKeyModel)
 auditlog.register(UUIDPrimaryKeyModel)
 auditlog.register(ModelPrimaryKeyModel)
@@ -485,3 +492,4 @@ auditlog.register(
     mask_fields=["credit_card"],
     mask_callable="auditlog_tests.test_app.mask.custom_mask_str",
 )
+auditlog.register(NullableFieldModel)

auditlog_tests/test_use_json_for_changes.py

@@ -1,5 +1,5 @@
 from django.test import TestCase, override_settings
-from test_app.models import JSONModel, RelatedModel, SimpleModel
+from test_app.models import JSONModel, NullableFieldModel, RelatedModel, SimpleModel
 
 from auditlog.models import LogEntry
 from auditlog.registry import AuditlogModelRegistry
@@ -147,3 +147,50 @@ class JSONForChangesTest(TestCase):
             all(v[1] is None for k, v in changes_dict.items()),
             'all values in the changes dict should None, not "None"',
         )
+
+    @override_settings(AUDITLOG_STORE_JSON_CHANGES=False)
+    def test_nullable_field_with_none_not_logged(self):
+        self.test_auditlog.register_from_settings()
+
+        obj = NullableFieldModel.objects.create(time=None, optional_text=None)
+        changes_dict = obj.history.latest().changes_dict
+
+        # None → None should NOT be logged as a change
+        self.assertNotIn("time", changes_dict)
+        self.assertNotIn("optional_text", changes_dict)
+
+    @override_settings(AUDITLOG_STORE_JSON_CHANGES=False)
+    def test_nullable_field_with_value_logged(self):
+        self.test_auditlog.register_from_settings()
+
+        obj = NullableFieldModel.objects.create(optional_text="something")
+        changes_dict = obj.history.latest().changes_dict
+
+        # None → "something" should be logged
+        self.assertIn("optional_text", changes_dict)
+        self.assertEqual(changes_dict["optional_text"], ["None", "something"])
+
+    @override_settings(AUDITLOG_STORE_JSON_CHANGES=True)
+    def test_nullable_field_with_none_not_logged_json_mode(self):
+        self.test_auditlog.register_from_settings()
+
+        obj = NullableFieldModel.objects.create(time=None, optional_text=None)
+        changes_dict = obj.history.latest().changes_dict
+
+        # None → None should NOT be logged
+        self.assertNotIn("time", changes_dict)
+        self.assertNotIn("optional_text", changes_dict)
+
+    @override_settings(AUDITLOG_STORE_JSON_CHANGES=False)
+    def test_nullable_field_update_none_to_value(self):
+        self.test_auditlog.register_from_settings()
+
+        obj = NullableFieldModel.objects.create(optional_text=None)
+        obj.optional_text = "updated"
+        obj.save()
+
+        changes_dict = obj.history.latest().changes_dict
+
+        # None → "updated" should be logged
+        self.assertIn("optional_text", changes_dict)
+        self.assertEqual(changes_dict["optional_text"], ["None", "updated"])

auditlog_tests/tests.py

@@ -1358,7 +1358,7 @@ class RegisterModelSettingsTest(TestCase):
 
         self.assertTrue(self.test_auditlog.contains(SimpleExcludeModel))
         self.assertTrue(self.test_auditlog.contains(ChoicesFieldModel))
-        self.assertEqual(len(self.test_auditlog.get_models()), 33)
+        self.assertEqual(len(self.test_auditlog.get_models()), 34)
 
     def test_register_models_register_model_with_attrs(self):
         self.test_auditlog._register_models(