Django - Link Sub-instance to Inheritance Model

def save_to_finishing(self):
        finishing_product = FinishingProduct(product_ptr_id=self.pk)
        finishing_product.__dict__.update(self.__dict__)
        finishing_product.save()

Reference:
http://stackoverflow.com/questions/4064808/django-model-inheritance-create-sub-instance-of-existing-instance-downcast