public function dsql($class = null)
    {
        $class = $class ?: $this->dsql_class;
        $obj = $this->add($class);
        if (!$obj instanceof DB_dsql) {
            throw $this->exception('Specified class must be descendant of DB_dsql')
                ->addMoreInfo('class', $class);
        }

        return $obj;
    }