public static function hasManyForeignKey($attr)
    {
        if (!self::hasHasMany($attr)) {
            return null;
        }

        $cls     = get_called_class();
        $configs = self::$_has_many[$cls][$attr];
        $key     = is_array($configs) && array_key_exists("foreign_key", $configs) ? $configs["foreign_key"] : (self::isIgnoringCase() ? strtolower($cls)."_id" : $cls."_id");
        return $key;
    }