Fixed O(n) queries when adding M2M objects (#1333)

CHANGES.rst

@@ -27,6 +27,9 @@ Unreleased
   "Customizing the History Admin Templates" for overriding its template context (gh-1128)
 - Fixed the setting ``SIMPLE_HISTORY_ENABLED = False`` not preventing M2M historical
   records from being created (gh-1328)
+- For history-tracked M2M fields, adding M2M objects (using ``add()`` or ``set()``)
+  used to cause a number of database queries that scaled linearly with the number of
+  objects; this has been fixed to now be a constant number of queries (gh-1333)
 
 3.5.0 (2024-02-19)
 ------------------

simple_history/models.py

@@ -687,18 +687,21 @@ def create_historical_record_m2ms(self, history_instance, instance):
             m2m_history_model = self.m2m_models[field]
             original_instance = history_instance.instance
             through_model = getattr(original_instance, field.name).through
+            through_model_field_names = [f.name for f in through_model._meta.fields]
+            through_model_fk_field_names = [
+                f.name for f in through_model._meta.fields if isinstance(f, ForeignKey)
+            ]
 
             insert_rows = []
 
             through_field_name = utils.get_m2m_field_name(field)
             rows = through_model.objects.filter(**{through_field_name: instance})
+            rows = rows.select_related(*through_model_fk_field_names)
             for row in rows:
                 insert_row = {"history": history_instance}
 
-                for through_model_field in through_model._meta.fields:
-                    insert_row[through_model_field.name] = getattr(
-                        row, through_model_field.name
-                    )
+                for field_name in through_model_field_names:
+                    insert_row[field_name] = getattr(row, field_name)
                 insert_rows.append(m2m_history_model(**insert_row))
 
             pre_create_historical_m2m_records.send(

simple_history/tests/tests/test_models.py

@@ -2376,6 +2376,43 @@ def test_bulk_add_remove(self):
         historical_place = m2m_record.places.first()
         self.assertEqual(historical_place.place, self.place)
 
+    def test_add_remove_set_and_clear_methods_make_expected_num_queries(self):
+        for num_places in (1, 2, 4):
+            with self.subTest(num_places=num_places):
+                start_pk = 100 + num_places
+                places = Place.objects.bulk_create(
+                    Place(pk=pk, name=f"Place {pk}")
+                    for pk in range(start_pk, start_pk + num_places)
+                )
+                self.assertEqual(len(places), num_places)
+                self.assertEqual(self.poll.places.count(), 0)
+
+                # The number of queries should stay the same, regardless of
+                # the number of places added or removed
+                with self.assertNumQueries(5):
+                    self.poll.places.add(*places)
+                self.assertEqual(self.poll.places.count(), num_places)
+
+                with self.assertNumQueries(3):
+                    self.poll.places.remove(*places)
+                self.assertEqual(self.poll.places.count(), 0)
+
+                with self.assertNumQueries(6):
+                    self.poll.places.set(places)
+                self.assertEqual(self.poll.places.count(), num_places)
+
+                with self.assertNumQueries(4):
+                    self.poll.places.set([])
+                self.assertEqual(self.poll.places.count(), 0)
+
+                with self.assertNumQueries(5):
+                    self.poll.places.add(*places)
+                self.assertEqual(self.poll.places.count(), num_places)
+
+                with self.assertNumQueries(3):
+                    self.poll.places.clear()
+                self.assertEqual(self.poll.places.count(), 0)
+
     def test_m2m_relation(self):
         # Ensure only the correct M2Ms are saved and returned for history objects
         poll_2 = PollWithManyToMany.objects.create(question="Why", pub_date=today)