Sort ModelChange's old and new M2M lists

This should help prevent flaky tests, like the order of the `Place`
objects in
`AdminSiteTest.test_history_list_contains_diff_changes_for_m2m_fields()`.

CHANGES.rst

@@ -19,7 +19,8 @@ Unreleased
 - ``ModelDelta`` and ``ModelChange`` (in ``simple_history.models``) are now immutable
   dataclasses; their signatures remain unchanged (gh-1128)
 - ``ModelDelta``'s ``changes`` and ``changed_fields`` are now sorted alphabetically by
-  field name, to prevent flaky tests by making their order consistent (gh-1128)
+  field name. Also, if ``ModelChange`` is for an M2M field, its ``old`` and ``new``
+  lists are sorted by the related object. This should help prevent flaky tests. (gh-1128)
 - ``diff_against()`` has a new keyword argument, ``foreign_keys_are_objs``;
   see usage in the docs under "History Diffing" (gh-1128)
 - Added ``HistoricalQuerySet.select_related_history_tracked_objs()``; see usage in the

docs/history_diffing.rst

@@ -19,6 +19,7 @@ This will return a ``ModelDelta`` object with the following attributes:
 
     - For many-to-many fields, these values will be lists of dictionaries from the
       through model fields to the primary keys of the through model's related objects.
+      The lists are sorted by the value of the many-to-many related object.
 
 This may be useful when you want to construct timelines and need to get only
 the model modifications.

simple_history/models.py

@@ -45,7 +45,7 @@
     pre_create_historical_m2m_records,
     pre_create_historical_record,
 )
-from .utils import get_change_reason_from_object
+from .utils import get_change_reason_from_object, get_m2m_reverse_field_name
 
 try:
     from asgiref.local import Local as LocalContext
@@ -1047,9 +1047,12 @@ def _get_m2m_field_changes_for_diff(
         changes = []
 
         for field in m2m_fields:
-            old_m2m_manager = getattr(old_history, field)
-            new_m2m_manager = getattr(self, field)
-            m2m_through_model_opts = new_m2m_manager.model._meta
+            original_field_meta = self.instance_type._meta.get_field(field)
+            reverse_field_name = get_m2m_reverse_field_name(original_field_meta)
+            # Sort the M2M rows by the related object, to ensure a consistent order
+            old_m2m_qs = getattr(old_history, field).order_by(reverse_field_name)
+            new_m2m_qs = getattr(self, field).order_by(reverse_field_name)
+            m2m_through_model_opts = new_m2m_qs.model._meta
 
             # Create a list of field names to compare against.
             # The list is generated without the PK of the intermediate (through)
@@ -1060,8 +1063,8 @@ def _get_m2m_field_changes_for_diff(
                 for f in m2m_through_model_opts.fields
                 if f.editable and f.name not in ["id", "m2m_history_id", "history"]
             ]
-            old_rows = list(old_m2m_manager.values(*through_model_fields))
-            new_rows = list(new_m2m_manager.values(*through_model_fields))
+            old_rows = list(old_m2m_qs.values(*through_model_fields))
+            new_rows = list(new_m2m_qs.values(*through_model_fields))
 
             if old_rows != new_rows:
                 if foreign_keys_are_objs:
@@ -1073,7 +1076,7 @@ def _get_m2m_field_changes_for_diff(
 
                     # Set the through fields to their related model objects instead of
                     # the raw PKs from `values()`
-                    def rows_with_foreign_key_objs(m2m_manager):
+                    def rows_with_foreign_key_objs(m2m_qs):
                         def get_value(obj, through_field):
                             try:
                                 value = getattr(obj, through_field)
@@ -1094,11 +1097,11 @@ def get_value(obj, through_field):
                                 through_field: get_value(through_obj, through_field)
                                 for through_field in through_model_fields
                             }
-                            for through_obj in m2m_manager.select_related(*fk_fields)
+                            for through_obj in m2m_qs.select_related(*fk_fields)
                         ]
 
-                    old_rows = rows_with_foreign_key_objs(old_m2m_manager)
-                    new_rows = rows_with_foreign_key_objs(new_m2m_manager)
+                    old_rows = rows_with_foreign_key_objs(old_m2m_qs)
+                    new_rows = rows_with_foreign_key_objs(new_m2m_qs)
 
                 change = ModelChange(field, old_rows, new_rows)
                 changes.append(change)