我们以生活中一个老师教多个学生，一个学生有多个老师为例，讲解MybatisPlus如何实现多对多的映射关系。

1)创建实体 Teachers.java(教师类)

package com.oracle.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import java.io.Serializable;

@Entity
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teachers extends BaseEntity implements Serializable,Cloneable {

    @Id
    @GeneratedValue(generator="mytid")
    @GenericGenerator(name="mytid",strategy="assigned")
    @Column(length = 4)
    private String tid; //教师编号
    private String tname; //教师姓名
}

Students.java（学生类）

package com.oracle.entity;

import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableId;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import javax.persistence.*;
import java.io.Serializable;

@Data
@NoArgsConstructor
@AllArgsConstructor
@Entity //表示这个类对应数据库中的一张表
public class Students extends BaseEntity implements Serializable,Cloneable {

    //既然是表，就必须有主键
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY) //使用数据库默认的主键生成策略
    @TableId(type = IdType.AUTO)
    private int sid; //主键
    @Column(length = 20)  //规定学生的姓名不超过20个字符
    private String sname; //姓名，默认是占用255个字符。
    @Column(length = 2)
    private String gender; //性别
    @Column(length = 32)
    private String birthday; //出生日期
    private String school; //所在学校
    private String major; //所学的专业
    private boolean status; //状态  1：在籍  0：辍学

    @Column(length = 4)
    private String cid; //班级的编号
}

首先定义联合主键类。定义该联合主键的目的是方便今后实现Mybatis与JPA双数据源的封装。 例如：StudentsTeachersRepository定义如下

public interface StudentsTeachersRepository extends BaseRepository<StudentsTeachers, StudentsTeachersPrimaryKey> {

}

package com.oracle.entity;

import javax.persistence.Embeddable;
import javax.persistence.EmbeddedId;
import javax.persistence.IdClass;
import java.io.Serializable;

public class StudentsTeachersPrimaryKey implements Serializable {
    private int sid;
    private String tid;
}

StudentsTeachers(学生教师关系类)。

package com.oracle.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.IdClass;
import java.io.Serializable;


@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
@IdClass(StudentsTeachersPrimaryKey.class)
public class StudentsTeachers extends BaseEntity implements Serializable,Cloneable {
    //sid+tid构成了联合主键
    @Id
    protected int sid;
    @Id
    private String tid;
}

测试多对多的映射关系。由于篇幅限制Students和Teachers的Mapper和Service层代码省略，大家可以自行参考前面案例实现。

package com.oracle.service;

import com.oracle.entity.ClassRoom;
import com.oracle.entity.Students;
import com.oracle.entity.StudentsTeachers;
import com.oracle.entity.Teachers;
import com.oracle.view.StudentsView;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import javax.annotation.Resource;
import java.util.List;

@RunWith(SpringRunner.class)
@SpringBootTest
public class StudentsServiceTest {

    @Resource
    private ClassRoomService classRoomService;
    @Resource
    private StudentsService studentsService;

    @Resource
    private TeachersService teachersService;

    @Resource
    private StudentsTeachersService studentsTeachersService;

    @Test
    public void testSaveStudents(){

        ClassRoom classRoom = new ClassRoom("C001","四川工业学院软件工程1班");
        classRoomService.insertWithMybatis(classRoom);

        Teachers t1 = new Teachers("T001","刘老师");
        Teachers t2 = new Teachers("T002","陈老师");
        Teachers t3 = new Teachers("T003","赵老师");

        teachersService.insertWithMybatis(t1);
        teachersService.insertWithMybatis(t2);
        teachersService.insertWithMybatis(t3);


        Students s1 = new Students(1,"张三","男","1999-10-10","四川工业学院","软件工程",true,classRoom.getCid());
        Students s2 = new Students(2,"李四","女","2000-10-10","四川工业学院","软件工程",true,classRoom.getCid());
        Students s3 = new Students(3,"王五","男","1999-10-10","四川工业学院","软件工程",true,classRoom.getCid());

        studentsService.insertWithMybatis(s1);
        studentsService.insertWithMybatis(s2);
        studentsService.insertWithMybatis(s3);

        StudentsTeachers st1 = new StudentsTeachers(s1.getSid(),t1.getTid());
        StudentsTeachers st2 = new StudentsTeachers(s1.getSid(),t2.getTid());
        StudentsTeachers st3 = new StudentsTeachers(s1.getSid(),t3.getTid());
        StudentsTeachers st4 = new StudentsTeachers(s2.getSid(),t1.getTid());
        StudentsTeachers st5 = new StudentsTeachers(s2.getSid(),t2.getTid());
        StudentsTeachers st6 = new StudentsTeachers(s3.getSid(),t2.getTid());
        StudentsTeachers st7 = new StudentsTeachers(s3.getSid(),t3.getTid());
        //StudentsTeachers st1 = new StudentsTeachers(s1.getSid(),t1.getTid());

        studentsTeachersService.insertWithMybatis(st1);
        studentsTeachersService.insertWithMybatis(st2);
        studentsTeachersService.insertWithMybatis(st3);
        studentsTeachersService.insertWithMybatis(st4);
        studentsTeachersService.insertWithMybatis(st5);
        studentsTeachersService.insertWithMybatis(st6);
        studentsTeachersService.insertWithMybatis(st7);
    }

}