Add support for m2m changes in AbstractLogEntry.changes_str (#798)

* Add test case to test log entry changes_str property for m2m changes.

* Add support for m2m field changes and generic changes in AbstractLogEntry.changes_str property.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add more test cases for changes_str.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add chengelog note

* Validate type and length of changes_dict values.

* Restructure change iterator.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: marco-thirona

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Next Release
 
+#### Fixes
+
+- `KeyError` when calling `changes_str` on a log entry that tracks many-to-many field changes ([#798](https://github.com/jazzband/django-auditlog/pull/798))
+
 ## 3.4.1 (2025-12-13)
 
 #### Fixes

auditlog/models.py

@@ -427,21 +427,29 @@ def changes_str(self, colon=": ", arrow=" \u2192 ", separator="; "):
         not satisfying, please use :py:func:`LogEntry.changes_dict` and format the string yourself.
 
         :param colon: The string to place between the field name and the values.
-        :param arrow: The string to place between each old and new value.
+        :param arrow: The string to place between each old and new value (non-m2m field changes only).
         :param separator: The string to place between each field.
         :return: A readable string of the changes in this log entry.
         """
         substrings = []
 
-        for field, values in self.changes_dict.items():
-            substring = "{field_name:s}{colon:s}{old:s}{arrow:s}{new:s}".format(
-                field_name=field,
-                colon=colon,
-                old=values[0],
-                arrow=arrow,
-                new=values[1],
-            )
-            substrings.append(substring)
+        for field, value in sorted(self.changes_dict.items()):
+            if isinstance(value, (list, tuple)) and len(value) == 2:
+                # handle regular field change
+                substring = "{field_name:s}{colon:s}{old:s}{arrow:s}{new:s}".format(
+                    field_name=field,
+                    colon=colon,
+                    old=value[0],
+                    arrow=arrow,
+                    new=value[1],
+                )
+                substrings.append(substring)
+            elif isinstance(value, dict) and value.get("type") == "m2m":
+                # handle m2m change
+                substring = (
+                    f"{field}{colon}{value['operation']} {sorted(value['objects'])}"
+                )
+                substrings.append(substring)
 
         return separator.join(substrings)
 

auditlog_tests/tests.py

@@ -131,6 +131,11 @@ def check_update_log_entry(self, obj, history):
             {"boolean": ["False", "True"]},
             msg="The change is correctly logged",
         )
+        self.assertEqual(
+            history.changes_str,
+            "boolean: False → True",
+            msg="Changes string is correct",
+        )
 
     def test_update_specific_field_supplied_via_save_method(self):
         obj = self.obj
@@ -149,6 +154,11 @@ def test_update_specific_field_supplied_via_save_method(self):
                 "when using the `update_fields`."
             ),
         )
+        self.assertEqual(
+            obj.history.get(action=LogEntry.Action.UPDATE).changes_str,
+            "boolean: False → True",
+            msg="Changes string is correct",
+        )
 
     def test_django_update_fields_edge_cases(self):
         """
@@ -179,6 +189,11 @@ def test_django_update_fields_edge_cases(self):
             {"boolean": ["False", "True"], "integer": ["None", "1"]},
             msg="The 2 fields changed are correctly logged",
         )
+        self.assertEqual(
+            obj.history.get(action=LogEntry.Action.UPDATE).changes_str,
+            "boolean: False → True; integer: None → 1",
+            msg="Changes string is correct",
+        )
 
     def test_delete(self):
         """Deletion is logged correctly."""
@@ -494,6 +509,13 @@ def test_changes(self):
             },
         )
 
+    def test_changes_str(self):
+        self.obj.related.add(self.related)
+        log_entry = self.obj.history.first()
+        self.assertEqual(
+            log_entry.changes_str, f"related: add {[smart_str(self.related)]}"
+        )
+
     def test_adding_existing_related_obj(self):
         self.obj.related.add(self.related)
         log_entry = self.obj.history.first()
@@ -725,6 +747,11 @@ def test_specified_save_fields_are_ignored_if_not_included(self):
             {"label": ["Initial label", "New label"]},
             msg="Only the label was logged, regardless of multiple entries in `update_fields`",
         )
+        self.assertEqual(
+            obj.history.get(action=LogEntry.Action.UPDATE).changes_str,
+            "label: Initial label → New label",
+            msg="Changes string is correct",
+        )
 
     def test_register_include_fields(self):
         sim = SimpleIncludeModel(label="Include model", text="Looong text")
@@ -2061,6 +2088,11 @@ def test_update(self):
             {"json": ["{}", '{"quantity": "1"}']},
             msg="The change is correctly logged",
         )
+        self.assertEqual(
+            history.changes_str,
+            'json: {} → {"quantity": "1"}',
+            msg="Changes string is correct",
+        )
 
     def test_update_with_no_changes(self):
         """No changes are logged."""
@@ -2697,6 +2729,7 @@ def test_access_log(self):
         )
         self.assertIsNone(log_entry.changes)
         self.assertEqual(log_entry.changes_dict, {})
+        self.assertEqual(log_entry.changes_str, "")
 
 
 class SignalTests(TestCase):
@@ -3120,6 +3153,9 @@ def test_use_base_manager_setting_changes(self):
                 }
             },
         )
+        self.assertEqual(
+            log_entry.changes_str, f"m2m_related: add {[smart_str(obj_two)]}"
+        )
 
 
 class TestMaskStr(TestCase):