编程改变世界（8）-- mybatis中的一对多映射

上次的文章中我们学习了一对一映射，这次我们来学习一对多映射
我们可以想象这样一个场景，一个班里会有很多学生，但是一名学生只属于一个班，这就是典型的一对多的关系
那我们就来建立一个学生表，tb_student，如下

image.png

依次建立pojo类，修改之前的classesMapper

package com.pojo;

public class Student {
    
    private int id;
    private String name;
    private int c_id;

    public Student() {
    }

    public Student(int id, String name, int c_id) {
        this.id = id;
        this.name = name;
        this.c_id = c_id;
    }

    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 int getC_id() {
        return c_id;
    }

    public void setC_id(int c_id) {
        this.c_id = c_id;
    }

    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", c_id=" + c_id + "]";
    }
    
    

}

第一种写法，使用ofType

<?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.classesMapper">

    <select id="findClassesById" resultMap="classesMap">
        select * from tb_class where id=#{id}
    </select>
    <select id="findClasses" resultMap="classesMap">
        select * from tb_class c
        inner join tb_teacher t on t.id = c.t_id
        inner join tb_student s on s.c_id = c.id
    </select>
    <resultMap id="classesMap" type="Classes">
        <id property="id" column="id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" select="mapper.teacherMapper.findTeacherById" column="t_id"/>
        <collection property="students" ofType="Student">
            <id property="id" column="sid"/>
            <result property="name" column="name"/>
        </collection>

    </resultMap>

</mapper>

第二种写法，使用select

<?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.classesMapper">

    <select id="findClassesById" resultMap="classesMap">
        select * from tb_class where id=#{id}
    </select>
    <select id="findClasses" resultMap="classesMap">
        select * from tb_class c
        inner join tb_teacher t on t.id = c.t_id
        inner join tb_student s on s.c_id = c.id
    </select>
    <resultMap id="classesMap" type="Classes">
        <id property="id" column="id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" select="mapper.teacherMapper.findTeacherById" column="t_id"/>
        <collection property="students" select="mapper.studentMapper.findStudentByClassesId" column="id"/>
    </resultMap>

</mapper>

这时就要加上studentMapper

<?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>
</mapper>

最后我们进行测试

package com.test;


import com.dao.ClassesDao;
import com.pojo.Classes;

import java.util.List;

public class Test {

    public static void main(String[] args) {

        ClassesDao classesDao = new ClassesDao();
        List<Classes> list = classesDao.findClasses();
        int i = 0;
        for (Classes classes:list
             ) {
            System.out.println("班级名称："+classes.getName()+" 教师名称"+classes.getTeacher().getName()+" 学生名称"+classes.getStudents().get(i).getName());
            i++;
        }


    }
}

输出结果

image.png