MANY_TO_MANYphp多对多表的链接返回的数组role=null为什么

namespace Admin\Model;
use Think\Model\RelationModel;

class UserModel extends RelationModel {

 Protected $tableName = 'user';//定义主表名称 //定义关联关系 Protected $_link=array(    'role' =>array(        'mapping_type' =>MANY_TO_MANY,        'foreign_key' =>'user_id',//主表外联        'relation_key' =>'role_id',//副表外联        'relation_table' =>'tp_role_user'//中间表        )    );

}
?>

namespace Admin\Controller;
use Think\Controller;
use Home\Model\UserModel;
use Think\Model\RelationModel;
class RbacController extends CommonController {
public function index(){
$result=D('User')->relation(true)->select();
var_dump($result);
die();
$this->display();
}