Use django builtin for determining through field name (#1218)

Fixes #1126

AUTHORS.rst

@@ -90,6 +90,7 @@ Authors
 - Lucas Wiman
 - Maciej "RooTer" Urbański
 - Marcelo Canina (`marcanuy <https://github.com/marcanuy>`_)
+- Marco Sirabella
 - Mark Davidoff
 - Martin Bachwerk
 - Marty Alchin

CHANGES.rst

@@ -24,6 +24,8 @@ Unreleased
   ``HistoricalRecords.context.request``) under some circumstances (gh-1188)
 - Made ``HistoryRequestMiddleware`` async-capable (gh-1209)
 - Fixed error when setting ``table_name`` with ``inherit=True`` (gh-1195)
+- Fixed ``FieldError`` when creating historical records for many-to-many fields with
+  ``to="self"`` (gh-1218)
 
 3.3.0 (2023-03-08)
 ------------------

simple_history/models.py

@@ -670,7 +670,8 @@ def create_historical_record_m2ms(self, history_instance, instance):
 
             insert_rows = []
 
-            through_field_name = type(original_instance).__name__.lower()
+            # `m2m_field_name()` is part of Django's internal API
+            through_field_name = field.m2m_field_name()
 
             rows = through_model.objects.filter(**{through_field_name: instance})
 

simple_history/tests/models.py

@@ -200,6 +200,11 @@ class PollChildRestaurantWithManyToMany(PollParentWithManyToMany):
     _history_m2m_fields = [restaurants]
 
 
+class PollWithSelfManyToMany(models.Model):
+    relations = models.ManyToManyField("self")
+    history = HistoricalRecords(m2m_fields=[relations])
+
+
 class CustomAttrNameForeignKey(models.ForeignKey):
     def __init__(self, *args, **kwargs):
         self.attr_name = kwargs.pop("attr_name", None)

simple_history/tests/tests/test_models.py

@@ -103,6 +103,7 @@
     PollWithManyToManyCustomHistoryID,
     PollWithManyToManyWithIPAddress,
     PollWithNonEditableField,
+    PollWithSelfManyToMany,
     PollWithSeveralManyToMany,
     Province,
     Restaurant,
@@ -1869,6 +1870,17 @@ def test_separation(self):
         self.assertEqual(add.restaurants.all().count(), 0)
         self.assertEqual(add.places.all().count(), 0)
 
+    def test_self_field(self):
+        poll1 = PollWithSelfManyToMany.objects.create()
+        poll2 = PollWithSelfManyToMany.objects.create()
+
+        self.assertEqual(poll1.history.all().count(), 1)
+
+        poll1.relations.add(poll2)
+        self.assertIn(poll2, poll1.relations.all())
+
+        self.assertEqual(poll1.history.all().count(), 2)
+
 
 class ManyToManyWithSignalsTest(TestCase):
     def setUp(self):