Simplified DeferredAttribute.__get__() a bit.
@@ -141,7 +141,7 @@ class DeferredAttribute:
return self
data = instance.__dict__
field_name = self.field.attname
- if data.get(field_name, self) is self:
+ if field_name not in data:
# Let's see if the field is part of the parent chain. If so we
# might be able to reuse the already loaded value. Refs #18343.
val = self._check_parent_chain(instance)
