fix(diff_against): support diffing against deleted records

AUTHORS.rst

@@ -43,6 +43,7 @@ Authors
 - Dmytro Shyshov (`xahgmah <https://github.com/xahgmah>`_)
 - Edouard Richard (`vied12 <https://github.com/vied12>` _)
 - Eduardo Cuducos
+- Eric Uriostigue (`euriostigue <https://github.com/euriostigue>`_)
 - Erik van Widenfelt (`erikvw <https://github.com/erikvw>`_)
 - Fábio Capuano (`fabiocapsouza <https://github.com/fabiocapsouza`_)
 - Filipe Pina (@fopina)

CHANGES.rst

@@ -3,6 +3,7 @@ Changes
 
 Unreleased
 ----------
+- Fixed `diff_against` to work with deleted objects (gh-1312)
 
 
 3.5.0 (2024-02-19)

simple_history/models.py

@@ -952,8 +952,8 @@ def diff_against(self, old_history, excluded_fields=None, included_fields=None):
         current_values = model_to_dict(self, fields=fields)
 
         for field in fields:
-            old_value = old_values[field]
-            current_value = current_values[field]
+            old_value = None if old_history.history_type == "-" else old_values[field]
+            current_value = None if self.history_type == "-" else current_values[field]
 
             if old_value != current_value:
                 changes.append(ModelChange(field, old_value, current_value))

simple_history/tests/tests/test_models.py

@@ -3,7 +3,6 @@
 import warnings
 from datetime import datetime, timedelta
 
-import django
 from django.apps import apps
 from django.conf import settings
 from django.contrib.auth import get_user_model
@@ -28,7 +27,6 @@
     pre_create_historical_m2m_records,
     pre_create_historical_record,
 )
-from simple_history.tests.custom_user.models import CustomUser
 from simple_history.tests.tests.utils import (
     database_router_override_settings,
     database_router_override_settings_history_in_diff_db,
@@ -72,7 +70,6 @@
     HistoricalCustomFKError,
     HistoricalPoll,
     HistoricalPollWithHistoricalIPAddress,
-    HistoricalPollWithManyToMany_places,
     HistoricalState,
     InheritedRestaurant,
     Library,
@@ -85,8 +82,6 @@
     ModelWithMultipleNoDBIndex,
     ModelWithSingleNoDBIndexUnique,
     MultiOneToOne,
-    MyOverrideModelNameRegisterMethod1,
-    OverrideModelNameAsCallable,
     OverrideModelNameUsingBaseModel1,
     Person,
     Place,
@@ -800,6 +795,33 @@ 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_deletion_record(self):
+        question = "what's up?"
+        p = Poll.objects.create(question=question, pub_date=today)
+        poll_pk = p.pk
+        new_record = p.history.first()
+        p.delete()
+
+        deletion_record = HistoricalPoll.objects.get(id=poll_pk, history_type="-")
+
+        with self.assertNumQueries(0):
+            delta = new_record.diff_against(
+                deletion_record, included_fields=["question"]
+            )
+            self.assertEqual(delta.changed_fields, ["question"])
+            self.assertEqual(len(delta.changes), 1)
+            self.assertEqual(delta.changes[0].new, question)
+            self.assertEqual(delta.changes[0].old, None)
+
+        with self.assertNumQueries(0):
+            delta = deletion_record.diff_against(
+                new_record, included_fields=["question"]
+            )
+            self.assertEqual(delta.changed_fields, ["question"])
+            self.assertEqual(len(delta.changes), 1)
+            self.assertEqual(delta.changes[0].new, None)
+            self.assertEqual(delta.changes[0].old, question)
+
 
 class GetPrevRecordAndNextRecordTestCase(TestCase):
     def assertRecordsMatch(self, record_a, record_b):