save custom field data

netbox/circuits/tests/test_models.py

@@ -8,6 +8,8 @@ from circuits.models import Circuit, CircuitTermination, CircuitType, Provider,
 from core.choices import ObjectChangeActionChoices
 from core.models import ObjectChange
 from dcim.models import Location, Region, Site, SiteGroup
+from extras.choices import CustomFieldStatusChoices, CustomFieldTypeChoices
+from extras.models import CustomField
 from netbox.context_managers import event_tracking
 from users.models import User
 
@@ -573,3 +575,31 @@ class CircuitTerminationChangeLoggingTestCase(TestCase):
         self._tracked(self.circuits[0].delete)
 
         self.assertFalse(self._circuit_changes(self.circuits[0]).exists())
+
+    @tag('regression')  # Ref: #23134
+    def test_pointer_update_persists_populated_custom_field_defaults(self):
+        # The pointer is written with update_fields, but CustomFieldsMixin.save() populates defaults
+        # into custom_field_data, which the change log serializes. Both must reach the database.
+        custom_field = CustomField.objects.create(
+            name='probe_field',
+            type=CustomFieldTypeChoices.TYPE_TEXT,
+            default='default-value',
+            status=CustomFieldStatusChoices.STATUS_ACTIVE,
+        )
+        custom_field.object_types.set([ContentType.objects.get_for_model(Circuit)])
+        CustomField.objects.clear_cache()
+        Circuit.objects.filter(pk=self.circuits[0].pk).update(custom_field_data={})
+        ObjectChange.objects.all().delete()
+
+        self._tracked(lambda: CircuitTermination.objects.create(
+            circuit=self.circuits[0], term_side='A', termination=self.sites[0],
+        ))
+
+        self.circuits[0].refresh_from_db()
+        self.assertEqual(self.circuits[0].custom_field_data, {'probe_field': 'default-value'})
+
+        changes = self._circuit_changes(self.circuits[0])
+        self.assertEqual(changes.count(), 1)
+        self.assertEqual(
+            changes[0].postchange_data['custom_fields'], self.circuits[0].custom_field_data
+        )

netbox/netbox/models/features.py

@@ -347,9 +347,16 @@ class CustomFieldsMixin(models.Model):
         # covers fields still being provisioned as well as active ones, so that an object created
         # while a new field is being backfilled does not miss its default (see
         # CustomFieldManager.get_defaults_for_model()).
+        populated = False
         for name, default in CustomField.objects.get_defaults_for_model(self).items():
             if name not in self.custom_field_data:
                 self.custom_field_data[name] = default
+                populated = True
+
+        # A partial save must still write any default populated above, or it would reach the change
+        # log (which serializes the instance) without ever reaching the database.
+        if populated and kwargs.get('update_fields') is not None:
+            kwargs['update_fields'] = [*kwargs['update_fields'], 'custom_field_data']
 
         super().save(*args, **kwargs)