角色表
+--------+----------------------+------+-----+---------+----------------+
| Field  | Type                 | Null | Key | Default | Extra          |
+--------+----------------------+------+-----+---------+----------------+
| id     | smallint(6) unsigned | NO   | PRI | NULL    | auto_increment |
| name   | varchar(20)          | NO   |     | NULL    |                |
| pid    | smallint(6)          | YES  | MUL | NULL    |                |
| status | tinyint(1) unsigned  | YES  | MUL | NULL    |                |
| remark | varchar(255)         | YES  |     | NULL    |                |
+--------+----------------------+------+-----+---------+----------------++---------+-----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-----------------------+------+-----+---------+-------+
| role_id | mediumint(9) unsigned | YES | MUL | NULL | |
| user_id | char(32) | YES | MUL | NULL | |
+---------+-----------------------+------+-----+---------+-------+中间表
+---------+-----------------------+------+-----+---------+-------+
| Field   | Type                  | Null | Key | Default | Extra |
+---------+-----------------------+------+-----+---------+-------+
| role_id | mediumint(9) unsigned | YES  | MUL | NULL    |       |
| user_id | char(32)              | YES  | MUL | NULL    |       |
+---------+-----------------------+------+-----+---------+-------+用户表
+-----------+-------------+------+-----+---------+----------------+
| Field     | Type        | Null | Key | Default | Extra          |
+-----------+-------------+------+-----+---------+----------------+
| id        | int(11)     | NO   | PRI | NULL    | auto_increment |
| uname     | varchar(20) | YES  |     | NULL    |                |
| password  | varchar(40) | YES  |     | NULL    |                |
| logintime | date        | YES  |     | NULL    |                |
| loginip   | varchar(20) | YES  |     | NULL    |                |
| locks     | int(11)     | YES  |     | NULL    |                |
+-----------+-------------+------+-----+---------+----------------+<?php
namespace Admin\Model;
use Think\Model\RelationModel;

class UserModel extends RelationModel{
protected $tableName="user";
    protected $_link=array(
    "think_role"=>array(
     "mapping_type" => MANY_TO_MANY,
    'relation_table' => 'think_role_user',
    
     'mapping_name'=>'think_role',
    'foreign_key' => 'user_id',
    
    'relation_key' => 'role_id',
    
    "mapping_field" =>"id,name,remark"
    )
    );
}
public function user(){
    
        $user=D("user")->relation(true)->select();
        
        $this->display();
    }  [0] => array(7) {
    ["id"] => string(1) "1"
    ["uname"] => string(4) "dada"
    ["password"] => string(32) "e10adc3949ba59abbe56e057f20f883e"
    ["logintime"] => string(10) "0000-00-00"
    ["loginip"] => string(9) "127.0.0.1"
    ["locks"] => NULL
    ["think_role"] => NULL
  }
  [1] => array(7) {
    ["id"] => string(1) "2"
    ["uname"] => string(6) "sdadas"
    ["password"] => string(32) "e10adc3949ba59abbe56e057f20f883e"
    ["logintime"] => string(10) "0000-00-00"
    ["loginip"] => string(9) "127.0.0.1"
    ["locks"] => NULL
    ["think_role"] => NULL
  }附表think_role=>null，这是怎么回事？？？？？？？？？？？