@abstractmethod
    def setstate(self, state):
        raise NotImplementedError("Subclass of 'Optimizer' must implement 'setstate(state)'.")
Foreign keys can cross databases if they two databases have a common source
dataset/ETHPy150Open SMART-Lab/smartlearner/smartlearner/interfaces/optimizer.py/Optimizer.setstate--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.4/tests/regressiontests/multiple_database/tests.py/RouterTestCase.test_foreign_key_cross_database_protection