Use attname to get pk value (#650)

* Use `attname` to get pk value

* Add tests for model as primary key

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add clarifying comments to _get_pk_value

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

auditlog/models.py

@@ -213,12 +213,13 @@ class LogEntryManager(models.Manager):
         :type instance: Model
         :return: The primary key value of the given model instance.
         """
-        pk_field = instance._meta.pk.name
+        # Should be equivalent to `instance.pk`.
+        pk_field = instance._meta.pk.attname
         pk = getattr(instance, pk_field, None)
 
         # Check to make sure that we got a pk not a model object.
-        if isinstance(pk, models.Model):
-            pk = self._get_pk_value(pk)
+        # Should be guaranteed as we used `attname` above, not `name`.
+        assert not isinstance(pk, models.Model)
         return pk
 
     def _get_serialized_data_or_none(self, instance):

auditlog_tests/models.py

@@ -57,6 +57,26 @@ class UUIDPrimaryKeyModel(models.Model):
     history = AuditlogHistoryField(delete_related=True, pk_indexable=False)
 
 
+class ModelPrimaryKeyModel(models.Model):
+    """
+    A model with another model as primary key.
+    """
+
+    key = models.OneToOneField(
+        "SimpleModel",
+        primary_key=True,
+        on_delete=models.CASCADE,
+        related_name="reverse_primary_key",
+    )
+
+    text = models.TextField(blank=True)
+    boolean = models.BooleanField(default=False)
+    integer = models.IntegerField(blank=True, null=True)
+    datetime = models.DateTimeField(auto_now=True)
+
+    history = AuditlogHistoryField(delete_related=True, pk_indexable=False)
+
+
 class ProxyModel(SimpleModel):
     """
     A model that is a proxy for another model.
@@ -338,6 +358,7 @@ class AutoManyRelatedModel(models.Model):
 
 auditlog.register(AltPrimaryKeyModel)
 auditlog.register(UUIDPrimaryKeyModel)
+auditlog.register(ModelPrimaryKeyModel)
 auditlog.register(ProxyModel)
 auditlog.register(RelatedModel)
 auditlog.register(ManyRelatedModel)

auditlog_tests/tests.py

@@ -20,7 +20,7 @@ from django.db import models
 from django.db.models import JSONField, Value
 from django.db.models.functions import Now
 from django.db.models.signals import pre_save
-from django.test import RequestFactory, TestCase, override_settings
+from django.test import RequestFactory, TestCase, TransactionTestCase, override_settings
 from django.urls import resolve, reverse
 from django.utils import dateformat, formats
 from django.utils import timezone as django_timezone
@@ -46,6 +46,7 @@ from auditlog_tests.models import (
     JSONModel,
     ManyRelatedModel,
     ManyRelatedOtherModel,
+    ModelPrimaryKeyModel,
     NoDeleteHistoryModel,
     NullableJSONModel,
     PostgresArrayFieldModel,
@@ -332,6 +333,41 @@ class UUIDPrimaryKeyModelModelWithActorTest(
     pass
 
 
+class ModelPrimaryKeyModelBase(SimpleModelTest):
+    def make_object(self):
+        self.key = super().make_object()
+        return ModelPrimaryKeyModel.objects.create(key=self.key, text="I am strange.")
+
+
+class ModelPrimaryKeyModelTest(NoActorMixin, ModelPrimaryKeyModelBase):
+    pass
+
+
+class ModelPrimaryKeyModelWithActorTest(WithActorMixin, ModelPrimaryKeyModelBase):
+    pass
+
+
+# Must inherit from TransactionTestCase to use self.assertNumQueries.
+class ModelPrimaryKeyTest(TransactionTestCase):
+    def test_get_pk_value(self):
+        """
+        Test that the primary key can be retrieved without additional database queries.
+        """
+        key = SimpleModel.objects.create(text="I am not difficult.")
+        obj = ModelPrimaryKeyModel.objects.create(key=key, text="I am strange.")
+        # Refresh the object so the primary key object is not cached.
+        obj.refresh_from_db()
+        with self.assertNumQueries(0):
+            pk = LogEntry.objects._get_pk_value(obj)
+        self.assertEqual(pk, obj.pk)
+        self.assertEqual(pk, key.pk)
+        # Sanity check: verify accessing obj.key causes database access.
+        with self.assertNumQueries(1):
+            pk = obj.key.pk
+        self.assertEqual(pk, obj.pk)
+        self.assertEqual(pk, key.pk)
+
+
 class ProxyModelBase(SimpleModelTest):
     def make_object(self):
         return ProxyModel.objects.create(text="I am not what you think.")
@@ -1206,7 +1242,7 @@ class RegisterModelSettingsTest(TestCase):
 
         self.assertTrue(self.test_auditlog.contains(SimpleExcludeModel))
         self.assertTrue(self.test_auditlog.contains(ChoicesFieldModel))
-        self.assertEqual(len(self.test_auditlog.get_models()), 26)
+        self.assertEqual(len(self.test_auditlog.get_models()), 27)
 
     def test_register_models_register_model_with_attrs(self):
         self.test_auditlog._register_models(