compare only specific fields when diffing between m2m relations

Author: thijskramer

simple_history/models.py

@@ -901,9 +901,29 @@ def diff_against(self, old_history, excluded_fields=None, included_fields=None):
                 changes.append(ModelChange(field, old_value, current_value))
                 changed_fields.append(field)
 
+        # Separately compare m2m fields:
         for field in old_history._history_m2m_fields:
-            old_rows = list(getattr(old_history, field.name).values_list())
-            new_rows = list(getattr(self, field.name).values_list())
+            # First retrieve a single item to get the field names from:
+            reference_history_m2m_item = (
+                getattr(old_history, field.name).first()
+                or getattr(self, field.name).first()
+            )
+            history_field_names = []
+            if reference_history_m2m_item:
+                # Create a list of field names to compare against.
+                # The list is generated without the primary key of the intermediate
+                # table, the foreign key to the history record, and the actual 'history'
+                # field, to avoid false positives while diffing.
+                history_field_names = [
+                    f.name
+                    for f in reference_history_m2m_item._meta.fields
+                    if f.editable and f.name not in ["id", "m2m_history_id", "history"]
+                ]
+
+            old_rows = list(
+                getattr(old_history, field.name).values(*history_field_names)
+            )
+            new_rows = list(getattr(self, field.name).values(*history_field_names))
 
             if old_rows != new_rows:
                 change = ModelChange(field.name, old_rows, new_rows)

simple_history/tests/tests/test_models.py

@@ -2009,7 +2009,9 @@ def test_diff_against(self):
         add_record, create_record = self.poll.history.all()
 
         delta = add_record.diff_against(create_record)
-        expected_change = ModelChange("places", [], [(1, 1, 2, 1, 1)])
+        expected_change = ModelChange(
+            "places", [], [{"pollwithmanytomany": 1, "place": 1}]
+        )
         self.assertEqual(delta.changed_fields, ["places"])
         self.assertEqual(delta.old_record, create_record)
         self.assertEqual(delta.new_record, add_record)
@@ -2027,7 +2029,9 @@ def test_diff_against(self):
 
         # Second and third should have the same diffs as first and second, but with
         # old and new reversed
-        expected_change = ModelChange("places", [(1, 1, 2, 1, 1)], [])
+        expected_change = ModelChange(
+            "places", [{"place": 1, "pollwithmanytomany": 1}], []
+        )
         delta = del_record.diff_against(add_record)
         self.assertEqual(delta.changed_fields, ["places"])
         self.assertEqual(delta.old_record, add_record)