ThinkPHP视图模型指定数据库无效?

<?php

namespace Common\Model;

/**
 * 无法使用
 */
class MessageViewModel extends \Think\Model\ViewModel {

    protected $connection = "DB_CONF_MAN";

    protected $viewFields = array(
        "Message" => array(
            "*",
            "_type" => "LEFT",
        ),
        "MessageText" => array(
            "title",
            "content",
            "_on" => "Message.message_text_id = MessageText.id",
        )
    );

}