编程改变世界（7）mybatis中的一对一映射

上一次我们入门了mybatis的单表查询，这一次我们学习mybatis的高级映射中最简单的一对一映射
我们想象这样一个一对一的场景，一个班级对应一个班主任，那么这就是一种一对一的关系
上次我们已经建立了tb_teacher表，这次我们再建立一张tb——classes表

image.png
这个表中的t_id就是关联tb_teacher的id主键
我们来写相应的pojo类,其中要包含Teacher对象

package com.pojo;

import java.util.List;

public class Classes {

    private int id;
    private String name;
    private Teacher teacher;

    public Classes() {
    }

    public Classes(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;

    }

    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 Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }


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

下一步写classesMapper（方法一）

<?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="findClasses" resultMap="classesMap">
        select * from tb_class c inner join tb_teacher t on t.id = c.t_id
    </select>
    <resultMap id="classesMap" type="Classes">
        <id property="id" column="id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="Teacher">
            <id property="id" column="id"/>
            <result property="name" column="c_name"/>
            <result property="role" column="role"/>
        </association>
    </resultMap>

</mapper>

下一步写classesMapper（方法二）
select中的值一定是按逐渐查询返回的一个对象的id，column中的值是外键的名称

<?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="findClasses" resultMap="classesMap">
        select * from tb_class c inner join tb_teacher t on t.id = c.t_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"/>
    </resultMap>

</mapper>

然后再写ClassesDao

package com.dao;

import com.pojo.Classes;
import com.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.util.List;

public class ClassesDao {
    private SqlSession sqlSession = MyBatisUtil.getSqlSession();
    private String sql = "mapper.classesMapper.";
    public List<Classes> findClasses(){
        sql +="findClasses";
        List<Classes> list = sqlSession.selectList(sql);
        return list;
    }
}

最后就是测试了

package com.test;


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


import java.util.List;

public class Test {

    public static void main(String[] args) {

        ClassesDao classesDao = new ClassesDao();
        List<Classes> list = classesDao.findClasses();
        for (Classes classes:list) {
            System.out.println("id="+classes.getId()+" name="+classes.getName()+" t_id="+classes.getTeacher().getId());
        }


    }
}

测试结果如下

image.png