introducing NoopChangedObject for performance optimization where applicable

src/c3nav/editor/models/changedobject.py

@@ -22,6 +22,13 @@ class ApplyToInstanceError(Exception):
     pass
 
 
+class NoopChangedObject:
+    pk = None
+    @classmethod
+    def apply_to_instance(cls, *args, **kwargs):
+        pass
+
+
 class ChangedObject(models.Model):
     changeset = models.ForeignKey('editor.ChangeSet', on_delete=models.CASCADE, verbose_name=_('Change Set'))
     created = models.DateTimeField(auto_now_add=True, verbose_name=_('created'))

src/c3nav/editor/models/changeset.py

@@ -17,7 +17,7 @@ from django.utils.timezone import make_naive
 from django.utils.translation import ugettext_lazy as _
 from django.utils.translation import ungettext_lazy
 
-from c3nav.editor.models.changedobject import ApplyToInstanceError, ChangedObject
+from c3nav.editor.models.changedobject import ApplyToInstanceError, ChangedObject, NoopChangedObject
 from c3nav.editor.tasks import send_changeset_proposed_notification
 from c3nav.editor.wrappers import ModelInstanceWrapper, ModelWrapper, is_created_pk
 from c3nav.mapdata.models import LocationSlug, MapUpdate
@@ -374,7 +374,7 @@ class ChangeSet(models.Model):
         r = tuple((pk, values[name]) for pk, values in self.updated_existing.get(model, {}).items() if name in values)
         return r
 
-    def get_changed_object(self, obj) -> ChangedObject:
+    def get_changed_object(self, obj, allow_noop=False) -> typing.Union[ChangedObject, typing.Type[NoopChangedObject]]:
         if isinstance(obj, ModelInstanceWrapper):
             obj = obj._obj
         model = obj.__class__
@@ -394,6 +394,9 @@ class ChangeSet(models.Model):
         if is_created_pk(pk):
             raise model.DoesNotExist
 
+        if allow_noop:
+            return NoopChangedObject
+
         return ChangedObject(changeset=self, model_class=model, existing_object_pk=pk)
 
     def get_created_object(self, model, pk, get_foreign_objects=False, allow_deleted=False):

src/c3nav/editor/wrappers.py

@@ -201,7 +201,7 @@ class ModelInstanceWrapper(BaseWrapper):
         super().__init__(*args, **kwargs)
         self._affected_by_changeset = False
         if self._obj.pk is not None:
-            changed_object = self._changeset.get_changed_object(self._obj)
+            changed_object = self._changeset.get_changed_object(self._obj, allow_noop=True)
             self._affected_by_changeset = changed_object.pk is not None
             changed_object.apply_to_instance(self)