Think\Db\Query::order PHP Method

    public function order($field, $order = null)
    {
        if (!empty($field)) {
            if (is_string($field)) {
                if (!empty($this->options['via'])) {
                    $field = $this->options['via'] . '.' . $field;
                }
                $field = empty($order) ? $field : [$field => $order];
            } elseif (!empty($this->options['via'])) {
                foreach ($field as $key => $val) {
                    if (is_numeric($key)) {
                        $field[$key] = $this->options['via'] . '.' . $val;
                    } else {
                        $field[$this->options['via'] . '.' . $key] = $val;
                        unset($field[$key]);
                    }
                }
            }
            $this->options['order'] = $field;
        }
        return $this;
    }