public function orderByFixed($index, $direction = 'desc')
    {
        if ($direction != 'desc') {
            $direction = 'asc';
        }

        if (is_array($index)) {
            $this->attributes['orderFixed'][] = $index;
        } else {
            $this->attributes['orderFixed'][] = [$index, $direction];
        }

        return $this;
    }