def get_db_prep_save(self, value, connection):
        if value == '' or value == None:
            return None
        else:
            return self.rel.get_related_field().get_db_prep_save(value,
                connection=connection)
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/db/models/fields/related.py/ForeignKey.get_db_prep_save/original