TP5 報(bào)錯(cuò)Db類找不到

TP5 找不到Db類,不知道為何.
這是AdminUser類

<?php
namespace app\admin\model;

use Think\Db;

class AdminUser extends \app\common\base\ModelBase
{
    public $_table = '';
    public function __construct()
    {
        parent::__construct();
        $this->table = Db::table('admin_user');
        //$this->_table = 'admin_user';

    }
}

這是ModelBase類

<?php
namespace app\common\base;

use Think\Db;

class ModelBase
{
    public $_error = array();

    public function __construct()
    {

    }



    /**
     * 得到表數(shù)據(jù)
     *
     * @param int   $w    1:計(jì)數(shù);  2:數(shù)據(jù)  3:計(jì)數(shù)&數(shù)據(jù)  4:一條數(shù)據(jù)
     * @param array $sql  array('sql_ct'=>'計(jì)數(shù)語句', 'sql'=>'數(shù)據(jù)語句')
     *
     * @return mixed
     */
    public function getData($w=0, $sql=array())
    {
        //只得到計(jì)數(shù)
        if($w === 1)
        {
            $ct = Db::query($sql['sql_ct']);
            return empty($ct) ? 0 : intval($ct[0]['tot']);
        }

        //只得到數(shù)據(jù)
        if($w === 2)
        {
            return Db::query($sql['sql']);
        }

        return array();
    }
}

下面是提示報(bào)錯(cuò)

求大神指點(diǎn)!