thinkphp操作mongodb的问题

<?php
/**
 * Description: MongoDB操作
 * Author: yangxiangming@live.com
 * Date: 2015/9/9
 * Time: 13:35
 */

namespace Bbsapi\Model;
use Think\Model\MongoModel;

class ExampleModel extends MongoModel {

}

<?php
/**
 * Description: MongoDB操作
 * Author: yangxiangming@live.com
 * Date: 2015/9/9
 * Time: 13:51
 */

namespace \Controller;
use Think\Model\ExampleModel;
class ExampleController extends ExampleModel{

    public function example(){
        $where['_id'] = '54dd9116e4b061818991ac7d';
        $model = D('Example');

        /** 查询 */
        $result = $model->where($where)->select();

        /** 添加 */
        $data['name'] = 'Example';
        ……
        $model->add($data);

        /** 更新 */
        $data['name'] = 'ExampleTmp';
        ……
        $model->where($where)->save($data);

        /** 删除 */
        $model->where($where)->delete();
    }
}