mybatis多对多关系

老师和班级的关系

一个老师可以教多个班级，老师和班级可以看作时一对多的关系。但是反过来，班级和老师的关系是多对一吗？很显然不是，在把班级当作主体来看，一个班级有多个老师来授课，老师和班级又可以看成是多对一的关系。像这种相互一对多，多对一的关系，我理解为多对多。

对于这种关系的数据库表格设计需要添加一个中间关联的表，老师、班级之间通过中间表相互关联起来

数据库表格部分(三张表tb_class、tb_teacher、tb_class_teacher)

SQL脚本

Create Table

CREATE TABLE `tb_class` (
  `id` int NOT NULL AUTO_INCREMENT,
  `code` varchar(18) DEFAULT NULL,
  `NAME` varchar(18) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8



Create Table

CREATE TABLE `tb_teacher` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(18) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8


Create Table

CREATE TABLE `tb_class_teacher` (
  `id` int NOT NULL AUTO_INCREMENT,
  `cid` int DEFAULT NULL,
  `tid` int DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `cid` (`cid`),
  KEY `tid` (`tid`),
  CONSTRAINT `tb_class_teacher_ibfk_1` FOREIGN KEY (`cid`) REFERENCES `tb_class` (`id`),
  CONSTRAINT `tb_class_teacher_ibfk_2` FOREIGN KEY (`tid`) REFERENCES `tb_teacher` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8

查询所有班级以及班级对应的授课老师

Mapper部分和Dao部分的编写

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="cn.liziy.dao.ClazzDao">
<!-- 查询所有班级信息以及班级所属的授课老师   -->
    <select id="selectAllClazzAndTeacher" resultMap="ClazzTeacherMap">
        SELECT
                c.id cid,
                c.code,
                c.name cname,
                t.id,
                t.name
        FROM tb_class c
        LEFT JOIN tb_class_teacher ct ON c.id = ct.cid
        LEFT JOIN tb_teacher t ON t.`id` = ct.tid
    </select>
    <resultMap id="ClazzTeacherMap" type="cn.liziy.entity.Clazz">
        <id property="id" column="cid"/>
        <id property="code" column="code"/>
        <id property="name" column="cname"/>
        <collection property="teachers" javaType="ArrayList"
                    ofType="cn.liziy.entity.Teacher">
            <id property="id" column="id"/>
            <id property="name" column="name"/>
        </collection>
    </resultMap>
</mapper>

package cn.liziy.dao;

import cn.liziy.entity.Clazz;

import java.util.List;

/**
 * @Author liziyang
 * @Date 22:23 2020/8/6
 * @Description 班级数据访问接口
 **/
public interface ClazzDao {

    /**
     * 查询所有班级信息以及班级所属授课老师信息
     * @return
     */
    List<Clazz> selectAllClazzAndTeacher();
}

service层

package cn.liziy.service;

import cn.liziy.entity.Clazz;

import java.util.List;

public interface ClazzService {
    /**
     * 查询所有班级信息以及班级所属授课老师信息
     * @return
     */
    List<Clazz> selectAllClazzAndTeacher();
}

package cn.liziy.service.impl;

import cn.liziy.dao.ClazzDao;
import cn.liziy.entity.Clazz;
import cn.liziy.service.ClazzService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

/**
 * @ClassName ClazzServiceImpl
 * @Author:Liziy
 * @Date 2020/8/6 22:40
 * @Description:
 **/
@Service
public class ClazzServiceImpl implements ClazzService {
    @Autowired
    ClazzDao clazzDao;

    @Override
    public List<Clazz> selectAllClazzAndTeacher() {
        return clazzDao.selectAllClazzAndTeacher();
    }
}

Controller层

package cn.liziy.controller;

import cn.liziy.entity.Clazz;
import cn.liziy.service.ClazzService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.ResponseBody;

import java.util.List;

/**
 * @ClassName ClazzController
 * @Author:Liziy
 * @Date 2020/8/6 22:37
 * @Description:
 **/
@Controller
public class ClazzController {
    @Autowired
    ClazzService clazzService;

    @GetMapping("/clazz_teacher/all")
    public @ResponseBody
    List<Clazz> Clazz_Teacher(){
        return clazzService.selectAllClazzAndTeacher();
    }
}

控制台日志

json数据

上图json查询出的所有班级以及班级的所有授课老师

反之也可以通过查询所有的老师以及老师授课的班级

mapper和Dao的编写

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="cn.liziy.dao.TeacherDao">
<!-- 根据所有老师信息以及老师对应的授课班级 -->
    <select id="selectAllTeacherAndClazz" resultMap="TeacherClazzMap">
        SELECT
                t.id,
                t.name,
                c.id cid,
                c.code,
                c.name cname
        FROM tb_teacher t
        LEFT JOIN tb_class_teacher ct ON t.id = ct.tid
        LEFT JOIN tb_class c ON c.id = ct.cid
    </select>
    <resultMap id="TeacherClazzMap" type="cn.liziy.entity.Teacher">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="clazzes" javaType="ArrayList"
                    ofType="cn.liziy.entity.Clazz">
            <id property="id" column="cid"/>
            <result property="code" column="code"/>
            <result property="name" column="cname"/>
        </collection>
    </resultMap>
</mapper>

package cn.liziy.dao;

import cn.liziy.entity.Teacher;

import java.util.List;

/**
 * @ClassName TeacherDao
 * @Author:Liziy
 * @Date 2020/8/7 16:33
 * @Description:
 **/
public interface TeacherDao {

    /**
     * 查询所有老师信息以及授课的班级
     * @return
     */
    List<Teacher> selectAllTeacherAndClazz();
}

Sercie层

package cn.liziy.service;

import cn.liziy.entity.Teacher;

import java.util.List;

public interface TeacherService {

    /**
     * 查询所有老师信息以及授课的班级
     * @return
     */
    List<Teacher> selectAllTeacherAndClazz();
}

package cn.liziy.service.impl;

import cn.liziy.dao.TeacherDao;
import cn.liziy.entity.Teacher;
import cn.liziy.service.TeacherService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import java.util.List;

/**
 * @ClassName TeacherServiceImpl
 * @Author:Liziy
 * @Date 2020/8/7 16:39
 * @Description:
 **/
@Service
public class TeacherServiceImpl implements TeacherService {
    @Autowired
    TeacherDao teacherDao;
    @Override
    public List<Teacher> selectAllTeacherAndClazz() {
        return teacherDao.selectAllTeacherAndClazz();
    }
}

Controller层

package cn.liziy.controller;

import cn.liziy.entity.Teacher;
import cn.liziy.service.TeacherService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import java.util.List;

/**
 * @ClassName TeacherController
 * @Author:Liziy
 * @Date 2020/8/7 16:40
 * @Description:
 **/
@Controller
public class TeacherController {
    @Autowired
    TeacherService teacherService;

    @GetMapping("teacher_clazz/all")
    public @ResponseBody
    List<Teacher> Tracher_Clazz(){
        return teacherService.selectAllTeacherAndClazz();
    }
}

控制台日志

json数据

对于没有任何授课的旗木卡卡西 clazzes显示为空

对于有多项授课班级的日向日足 clazzes的jsons如上图所示