编程改变世界（9）-- mybatis中的多对多映射

上次我们说了一对多查询，这次我们来说一下多对多查询
这样的场景大家一定不陌生，一名同学可以选多门课，而一门课又有多个同学选，这就是典型的多对多关系(多对多就可以理解为两个一对一的组合)
这次我们需要创建一个课程表tb_lesson表和一个选课表tb_ls,如下

image.png
image.png

然后建立相应的pojo类Lesson

package com.pojo;

import java.util.List;

public class Lesson {
    private int id;
    private String name;
    private List<Student> students;

    public Lesson() {
    }


    public Lesson(int id, String name, List<Student> students) {
        this.id = id;
        this.name = name;
        this.students = students;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

    @Override
    public String toString() {
        return "Lesson{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", students=" + students +
                '}';
    }
}

下一步写lessonMapper

<?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="mapper.lessonMapper">

    <select id="selectLesson" resultMap="lessonMap">
        select * from tb_student s
        inner join tb_ls ls on s.sid=ls.s_id
        inner join tb_lesson l on l.id=ls.l_id
    </select>
    
    <resultMap id="lessonMap" type="Lesson">
        <id column="id" property="id"/>
        <result column="lname" property="name"/>
        <collection property="students" ofType="Student">
            <id column="sid" property="sid"/>
            <result column="name" property="name"/>
        </collection>
    </resultMap>
    

</mapper>

测试类

package com.test;


import com.dao.ClassesDao;
import com.dao.LessonDao;
import com.pojo.Classes;
import com.pojo.Lesson;
import com.pojo.Student;

import java.util.List;

public class Test {

    public static void main(String[] args) {

        LessonDao lessonDao = new LessonDao();
        List<Lesson> list = lessonDao.findLesson();
        for (Lesson lesson : list) {
            System.out.println(lesson.getName());
            List<Student> students = lesson.getStudents();
            for (Student student : students) {
                System.out.println(student);
            }
        }
    }
}

image.png

如果以学生表为主表，则Student类改为

package com.pojo;

import java.util.List;

public class Student {
    
    private int sid;
    private String name;
    private Classes classes;
    private List<Lesson> lessons;

    public Student() {
    }


    public Student(int sid, String name, Classes classes, List<Lesson> lessons) {
        this.sid = sid;
        this.name = name;
        this.classes = classes;
        this.lessons = lessons;
    }

    public int getSid() {
        return sid;
    }

    public void setSid(int sid) {
        this.sid = sid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Classes getClasses() {
        return classes;
    }

    public void setClasses(Classes classes) {
        this.classes = classes;
    }

    public List<Lesson> getLessons() {
        return lessons;
    }

    public void setLessons(List<Lesson> lessons) {
        this.lessons = lessons;
    }

    @Override
    public String toString() {
        return "Student{" +
                "sid=" + sid +
                ", name='" + name + '\'' +
                ", classes=" + classes +
                ", lessons=" + lessons +
                '}';
    }
}

studentMapper改为

<?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="mapper.studentMapper">
    <select id="findStudentByClassesId" resultType="Student">
        select * from tb_student where c_id=#{id}
    </select>

    <select id="findAll" resultMap="studentMap">
        select * from tb_student s
        inner join tb_ls ls on s.sid=ls.s_id
        inner join tb_lesson l on l.id=ls.l_id
    </select>

    <resultMap id="studentMap" type="Student">
        <id column="sid" property="sid"/>
        <result column="name" property="name"/>
        <collection property="lessons" ofType="Lesson">
            <id column="id" property="id"/>
            <result column="lname" property="name"/>
        </collection>
    </resultMap>
</mapper>

测试类

package com.test;


import com.dao.ClassesDao;
import com.dao.LessonDao;
import com.dao.StudentDao;
import com.pojo.Classes;
import com.pojo.Lesson;
import com.pojo.Student;

import java.util.List;

public class Test {

    public static void main(String[] args) {


        StudentDao studentDao = new StudentDao();
        List<Student> students = studentDao.findStudent();
        for (Student student:students){
            System.out.println(student.getName());
            List<Lesson> lessons = student.getLessons();
            for (Lesson lesson:lessons){
                System.out.println(lesson);
            }
        }
    }
}

结果

image.png