public function addPermissionByName($permission_name)
    {
        $permission = static::$app['db']->connection()
            ->table('permissions')->where('name', '=', $permission_name)->first();

        if (! $permission)
            return new \RuntimeException('No permission with that name found.');

        if (is_array($permission))
            return $this->addPermissionById($permission['id']);
        elseif (is_object($permission))
            return $this->addPermissionById($permission->id);
        else
            throw new \UnexpectedValueException('Value returned not array or instance of BridgePermission.');
    }