thinkphp 3.2 视图模型 生成的SQL语句提示 1267错误 求指点~

<?phpnamespace Home\Model;use Think\Model\ViewModel;class PicViewModel extends  ViewModel{    public $viewFields =array(       'Picture'=> array('picTitle','picToken','picCategroy','picAuthor','picPostTime','_as'=>'Picture'),        'PictureAttachment' =>array('pictureid','pictureurl','creattime','_on'=>'Picture.picToken = PictureAttachment.pictureid','_as'=>'PictureAttachment'),    );}

  public function  Pic(){         $PicModel =D('PicView');            $s1= $PicModel->field('picTitle,picCategroy,picAuthor,picPostTime,pictureurl')->order('picPostTime desc')->select();        dump($PicModel->getLastSql());        dump($s1);    }

SELECT Picture.picTitle AS picTitle,Picture.picCategroy AS picCategroy,Picture.picAuthor AS picAuthor,Picture.picPostTime AS picPostTime,PictureAttachment.pictureurl AS pictureurl FROM xxx_picture Picture  JOIN xxx_picture_attachment PictureAttachment ON Picture.picToken = PictureAttachment.pictureid ORDER BY Picture.picPostTime desc