public function orderBy($orderBy)
    {
        if (is_array($orderBy) === true) {
            $this->orderBy = '';
            foreach ($orderBy as $key => $elem) {
                $this->orderBy .= $key . ' ' . $elem . ', ';
            }
            $this->orderBy = substr($this->orderBy, 0, -2);
        } else {
            $this->orderBy = $orderBy;
        }

        return $this;
    }