数据库查询返回错误的行

我正在研究一些使用codeigniter和内置querybuilder访问数据库的web代码。

我尝试从数据库中为当前用户加载数据

$userModel = $this->loadModel('ModelUser');
$name = $this->session->userdata('user');
$user = $userModel->getUser($name);

getUser的代码:

function getUser($username)
{
    $this->db->where('username',$username);
    $query = $this->db->get('tblusers',1);
    $res = $query->result();
    if ($query->num_rows() > 0)
    {
        log_message('debug','Got user. ID = '.$res[0]->id);
        foreach($res[0] as $key => $val)
        {
            $this->$key = $val;
        }
        return $this;
    }
    else {
        log_message('info','failed to find user '.$username);
        return NULL;
    }
}

这很好，除了当我让会话过期时，在这种情况下，我得到了另一个用户的详细信息。

这些是测试getUser的结果:

$userModel->getUser("Admin"); //Got user. ID = Admin_ID
$userModel->getUser("john");  //Got user. ID = John_ID
$userModel->getUser("");      //Failed to find user
$userModel->getUser(null);    //Failed to find user

当我以Admin身份登录时，然后让会话超时，顶部代码段记录消息:

Got user. ID = John_ID

我希望Got user. ID = Admin_ID或Failed to find user