[bug] better handling of empty values for audit log (#13134)

* [bug] better handling of empty values for audit log

* reverse unrelated commits

src/backend/InvenTree/audit/models.py

@@ -119,6 +119,20 @@ def drop_orphan_currency_changes(instance, changes: dict) -> dict:
     return result
 
 
+def is_unchanged_value(value) -> bool:
+    """Return True if a recorded [old, new] change pair does not represent a real change.
+
+    None and '' are treated as equivalent, as nullable string fields (e.g. Part.IPN)
+    may hold either value for "empty" - e.g. a NULL value saved from a form as ''.
+    """
+    if not (isinstance(value, (list, tuple)) and len(value) == 2):
+        return False
+
+    old, new = (None if v == '' else v for v in value)
+
+    return old == new
+
+
 def is_concrete_field(model, field_name: str) -> bool:
     """Return True if field_name is a concrete field on the model (or is unknown)."""
     try:
@@ -224,9 +238,10 @@ class AuditLogEntryManager(LogEntryManager):
     def log_create(self, instance, force_log: bool = False, **kwargs):
         """Create a new log entry for the provided instance.
 
-        For updates, fields whose recorded old and new values are identical are dropped.
-        These occur when the in-memory value differs from the database value only in type
-        (e.g. an empty FieldFile vs. an empty string), and would otherwise add noise to every save.
+        For updates, fields whose recorded old and new values are identical (or both empty,
+        i.e. None or '') are dropped. These occur when the in-memory value differs from the
+        database value only in type (e.g. an empty FieldFile vs. an empty string, or a NULL
+        value saved from a form as ''), and would otherwise add noise to every save.
 
         Updates which assign a query expression (e.g. F('completed') + 1) are deferred until
         after the save, so that the value actually written can be recorded
@@ -264,11 +279,7 @@ class AuditLogEntryManager(LogEntryManager):
                 changes = {
                     field: value
                     for field, value in changes.items()
-                    if not (
-                        isinstance(value, (list, tuple))
-                        and len(value) == 2
-                        and value[0] == value[1]
-                    )
+                    if not is_unchanged_value(value)
                 }
 
                 if not changes and not force_log:

src/backend/InvenTree/audit/tests.py

@@ -208,6 +208,39 @@ class AuditModelTest(AuditTestMixin, TestCase):
         self.assertNotIn('reference_int', entry.changes_dict)
         self.assertNotIn('metadata', entry.changes_dict)
 
+    def test_empty_values_equivalent(self):
+        """Changing a field between None and '' is not recorded as a change."""
+        Part.objects.filter(pk=self.part.pk).update(IPN=None)
+        self.part.refresh_from_db()
+
+        # Saving an empty value from a form ('') over NULL
+        self.part.IPN = ''
+        self.part.description = 'Updated'
+        self.part.save()
+
+        entry = self.entries_for(self.part, action=AuditLogEntry.Action.UPDATE).last()
+        self.assertEqual(entry.changes_dict, {'description': ['A part', 'Updated']})
+
+        # No entry is created if the only "change" is between empty values
+        self.part.IPN = None
+        self.part.save()
+
+        self.assertEqual(
+            self.entries_for(self.part, action=AuditLogEntry.Action.UPDATE).count(), 1
+        )
+
+        # A real change to or from an empty value is still recorded
+        self.part.IPN = 'IPN-001'
+        self.part.save()
+        self.part.IPN = ''
+        self.part.save()
+
+        entries = self.entries_for(self.part, action=AuditLogEntry.Action.UPDATE)
+        self.assertEqual(
+            [e.changes_dict['IPN'] for e in entries.order_by('pk')[1:]],
+            [[None, 'IPN-001'], ['IPN-001', '']],
+        )
+
     def test_parent_link(self):
         """Child entries are linked to the parent - and are retained after deletion."""
         line = PurchaseOrderLineItem.objects.create(