Make diffing more robust for polymorphic models (#784)

* Add failing test for diffing polymorphic model instances.

* Make diffing more robust for polymorphic models:

When working with polymorphic models, where a child model inherits from a
parent model, Django's pre_save signal may send model instances in a way
where the log_update() handler receives an instance of the child as the
`old` model, but an instance of the parent as the `new` model.

This leads to a `FieldDoesNotExist` error when a field that only exists on the
child was modified, and `get_field_value()` attempts look up that field on the
parent.

This change makes diffing polymorphic models more robust by considering this
case in `get_default_value()`. Changes to those child fields won't be tracked
in these cases, but at least `django-auditlog` won't prevent the model from
being saved.

CHANGELOG.md

@@ -7,6 +7,10 @@
 - Add `AUDITLOG_USE_BASE_MANAGER` setting to override default manager use ([#766](https://github.com/jazzband/django-auditlog/pull/766))
 - Drop 'Python 3.9' support ([#773](https://github.com/jazzband/django-auditlog/pull/773))
 
+#### Fixes
+
+- Make diffing more robust for polymorphic models ([#784](https://github.com/jazzband/django-auditlog/pull/784))
+
 ## 3.3.0 (2025-09-18)
 
 #### Improvements

auditlog/diff.py

@@ -3,7 +3,7 @@ from collections.abc import Callable
 from datetime import timezone
 
 from django.conf import settings
-from django.core.exceptions import ObjectDoesNotExist
+from django.core.exceptions import FieldDoesNotExist, ObjectDoesNotExist
 from django.db.models import NOT_PROVIDED, DateTimeField, ForeignKey, JSONField, Model
 from django.utils import timezone as django_timezone
 from django.utils.encoding import smart_str
@@ -74,7 +74,7 @@ def get_field_value(obj, field, use_json_for_changes=False):
         try:
             model_field = obj._meta.get_field(field.name)
             default = model_field.default
-        except AttributeError:
+        except (AttributeError, FieldDoesNotExist):
             default = NOT_PROVIDED
 
         if default is NOT_PROVIDED:

auditlog_tests/test_app/models.py

@@ -487,6 +487,7 @@ auditlog.register(AltPrimaryKeyModel)
 auditlog.register(UUIDPrimaryKeyModel)
 auditlog.register(ModelPrimaryKeyModel)
 auditlog.register(ProxyModel)
+auditlog.register(RelatedModelParent)
 auditlog.register(RelatedModel)
 auditlog.register(ManyRelatedModel)
 auditlog.register(ManyRelatedModel.recursive.through)

auditlog_tests/tests.py

@@ -47,6 +47,7 @@ from test_app.models import (
     NullableJSONModel,
     ProxyModel,
     RelatedModel,
+    RelatedModelParent,
     ReusableThroughRelatedModel,
     SecretM2MModel,
     SecretRelatedModel,
@@ -2131,6 +2132,27 @@ class ModelInstanceDiffTest(TestCase):
         model_instance_diff(simple2, simple1)
         model_instance_diff(simple1, simple2)
 
+    def test_diff_polymorphic_models(self):
+        """No error is raised when comparing parent/child for polymorphic models."""
+
+        # This tests that when a polymorphic model is compared to its parent,
+        # no FieldDoesNotExist errors are raised because those fields don't exist
+        # on the parent model.
+
+        # relation target
+        simple = SimpleModel()
+        simple.save()
+
+        # the parent model
+        related_parent = RelatedModelParent()
+        related_parent.save()
+
+        # the child model, with some fields that don't exist on the parent
+        related = RelatedModel(related=simple, one_to_one=simple)
+        related.save()
+
+        model_instance_diff(related, related_parent)
+
     def test_object_repr_related_deleted(self):
         """No error is raised when __str__() loads a related object that has been deleted."""
         simple = SimpleModel()