public function update()
    {
        // convert to condition.
        $args = func_get_args();
        $attributes = array_shift($args);
        $cond = call_user_func_array(array($this, 'toCondition'), $args);

        // to SQL.
        $sql = $cond->toUpdateSQL($attributes);
        var_dump($sql, $cond->getParams());

        // query.
        $sth = $this->query($sql, $cond->getParams());
        return $sth->isSuccess();
    }