@abc.abstractmethod
    def add_instance(self, usage):
        pass
Discard trailing non-relation field if extant.
dataset/ETHPy150Open BU-NU-CLOUD-SP16/Trusted-Platform-Module-nova/nova/compute/resources/base.py/Resource.add_instance--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/contrib/admin/util.py/remove_trailing_data_field