public function sortBy($field, $order = 'ASC')
    {
        //argument test
        Argument::i()
            //Argument 1 must be a string
            ->test(1, 'string')
            //Argument 2 must be a string
            ->test(2, 'string');
        
        $this->sortBy[] = $field . ' ' . $order;
        
        return $this;
    }