def get_multi_generation(self, tables, db='default'):
        """Takes a list of table names and returns an aggregate
        value for the generation"""
        generations = []
        for table in tables:
            generations.append(self.get_single_generation(table, db))
        key = self.keygen.gen_multi_key(generations, db)
        val = self.cache_backend.get(key, None, db)
        #if local.get('in_test', None): print force_bytes(val).ljust(32), key
        if val is None:
            val = self.keygen.random_generator()
            self.cache_backend.set(key, val, settings.MIDDLEWARE_SECONDS, db)
        return val
Takes a list of table names and returns an aggregate
        value for the generation