Keep Coding

Let's have a model with decorator:
def upgradable(model):
#classmethod
def upgrade(cls, base, **kwargs):
base.__class__ = cls
for attr, value in kwargs.items():
setattr(base, attr, value)
return base
setattr(model, "upgrade", upgrade)
return model
class Person(models.Model):
name = models.CharField(max_length=50)
def save(self, *args, **kwargs):
self.name = self.name.upper()
super(Person, self).save(*args, **kwargs)
and inherited model:
#upgradable
class User(Person):
email = models.EmailField();
and I'm trying to upgrade
person = Person.objects.get(...)
user = User.upgrade(person, email="example#example.com")
user.save()
But the error appears:
Save with update_fields did not affect any rows.
in connection to super(Person, self).save(*args, **kwargs) (in method save of model person)
It isn't total wrong, because indeed I don't want to change anything in table person, I just want to add something to table user (pointer to person and email).
So how to skip this warning?

The answer wasn't obvious - I tried to add
user.save(force_update = True)
but it isn't work.
But
user.save(force_insert = True)
do the job. Strange, but works.