Proper handling of custom m2m relationships

simple_history/models.py

@@ -660,6 +660,19 @@ def m2m_changed(self, instance, action, attr, pk_set, reverse, **_):
             # It should be safe to ~ this since the row must exist to modify m2m on it
             self.create_historical_record(instance, "~")
 
+    def _get_through_field_name(self, through_table_fields, model):
+        """
+        Find the name of the field in the through table. This is necessary for the
+        custom through tables where Django conventions don't apply.
+        """
+        foreign_keys = filter(
+            lambda model_field: isinstance(model_field, models.ForeignKey),
+            through_table_fields,
+        )
+        for field in foreign_keys:
+            if field.related_model == model:
+                return field.name
+
     def create_historical_record_m2ms(self, history_instance, instance):
         for field in history_instance._history_m2m_fields:
             m2m_history_model = self.m2m_models[field]
@@ -668,7 +681,10 @@ def create_historical_record_m2ms(self, history_instance, instance):
 
             insert_rows = []
 
-            through_field_name = type(original_instance).__name__.lower()
+            # find the name of the field in custom or default through table
+            through_field_name = self._get_through_field_name(
+                through_model._meta.fields, original_instance._meta.model
+            )
 
             rows = through_model.objects.filter(**{through_field_name: instance})
 

simple_history/tests/models.py

@@ -158,6 +158,28 @@ class PollWithSeveralManyToMany(models.Model):
     history = HistoricalRecords(m2m_fields=[places, restaurants, books])
 
 
+class Tag(models.Model):
+    name = models.CharField(max_length=100)
+
+
+class PollTags(models.Model):
+    poll = models.ForeignKey(
+        "PollParentWithManyToManyCustomThrough", on_delete=models.CASCADE
+    )
+    tag = models.ForeignKey(Tag, on_delete=models.CASCADE)
+
+
+class PollParentWithManyToManyCustomThrough(models.Model):
+    question = models.CharField(max_length=200)
+    pub_date = models.DateTimeField("date published")
+    tags = models.ManyToManyField(Tag, through=PollTags)
+
+    history = HistoricalRecords(
+        m2m_fields=[tags],
+        inherit=True,
+    )
+
+
 class PollParentWithManyToMany(models.Model):
     question = models.CharField(max_length=200)
     pub_date = models.DateTimeField("date published")

simple_history/tests/tests/test_models.py

@@ -94,6 +94,7 @@
     PollChildBookWithManyToMany,
     PollChildRestaurantWithManyToMany,
     PollInfo,
+    PollParentWithManyToManyCustomThrough,
     PollWithAlternativeManager,
     PollWithExcludedFieldsWithDefaults,
     PollWithExcludedFKField,
@@ -781,6 +782,12 @@ def test_history_with_unknown_field(self):
         with self.assertNumQueries(0):
             new_record.diff_against(old_record, excluded_fields=["unknown_field"])
 
+    def test_history_with_custom_through_field(self):
+        PollParentWithManyToManyCustomThrough.objects.create(
+            question="what's up?", pub_date=today
+        )
+        self.assertEqual(PollParentWithManyToManyCustomThrough.objects.count(), 1)
+
 
 class GetPrevRecordAndNextRecordTestCase(TestCase):
     def assertRecordsMatch(self, record_a, record_b):