MyBatis 一对一关系映射

两张表，学生表，地址表，学生表中的addressId对应地址表中的id。

首先构建两个实体类，Student，Address

package com.maya.model;

public class Student {
    private int id;
    private String name;
    private int age;
    private Address address;
    
    
    
    public Address getAddress() {
        return address;
    }
    public void setAddress(Address address) {
        this.address = address;
    }
    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 getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    
    public Student(int id, String name, int age) {
        super();
        this.id = id;
        this.name = name;
        this.age = age;
    }
    public Student(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    public Student() {
        super();
    }
    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", age=" + age + ", address=" + address + "]";
    }    
}

package com.maya.model;

public class Address {
    private int id;
    private String sheng;
    private String shi;
    private String qu;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getSheng() {
        return sheng;
    }
    public void setSheng(String sheng) {
        this.sheng = sheng;
    }
    public String getShi() {
        return shi;
    }
    public void setShi(String shi) {
        this.shi = shi;
    }
    public String getQu() {
        return qu;
    }
    public void setQu(String qu) {
        this.qu = qu;
    }
    @Override
    public String toString() {
        return "Address [id=" + id + ", sheng=" + sheng + ", shi=" + shi + ", qu=" + qu + "]";
    }    
}

接下来我们先介绍以下3种不太常用的方式（当然平时做些小项目的话还是很直接很暴力的）：

1.

　　<resultMap type="Student" id="StudentResult">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        
        <result property="address.id" column="addressId"/>
        <result property="address.sheng" column="sheng"/>
        <result property="address.shi" column="shi"/>
        <result property="address.qu" column="qu"/>
    </resultMap>

　　当然这是最笨的一种办法，通过成员属性的属性来对应其数据库的字段

2.

　　<resultMap type="Student" id="StudentResult">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <association property="address" javaType="Address">
            <result property="id" column="id"/>
            <result property="sheng" column="sheng"/>
            <result property="shi" column="shi"/>
            <result property="qu" column="qu"/>
        </association>
    </resultMap>
　　
　　通过直接在StudentMapper中的属性中定义association标签来实现
　　property是属性，javaTye是引用的类型，紧接着将Address的属性也全部配置进来

3.

　　<resultMap type="Address" id="AddressResult">
        <result property="id" column="id"/>
        <result property="sheng" column="sheng"/>
        <result property="shi" column="shi"/>
        <result property="qu" column="qu"/>
    </resultMap>
    
    <resultMap type="Student" id="StudentResult">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <association property="address" resultMap="AddressResult"/>
    </resultMap>

　　在studentMapper中定义两个resultMapper，其实这种方式就算不错了，但是如果别的mapper也需要Address呢？很显然这就不是很实用了。
　　当然这已经很接近我们常用的方法了。
　　最常用的就是，再定义一个mapper接口和mapper映射，通过association属性来引入，具体代码如下：

0.

首先定义Address的接口与映射文件

package com.maya.mappers;

import com.maya.model.Address;

public interface AddressMapper {
    //我们只需要定义这一个方法就ok，
    //因为当穿过来的Student的外键address时，这里接收的就是其主键值，这样我们就能查到student对应的地址了
    public Address getById(int id);
}

<?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="com.maya.mappers.AddressMapper">
    <!-- resultMap是要有的，大家都这么写，因为这样重用性高。用其它的方式当然也没错 -->
    <resultMap type="Address" id="AddressResult">
        <id property="id" column="id" />
        <result property="sheng" column="sheng" />
        <result property="shi" column="shi" />
        <result property="qu" column="qu" />
    </resultMap>
    
    <select id="getById" parameterType="int" resultMap="AddressResult">
        select * from address where id=#{id}
    </select>
    
</mapper>

下面是StudentMapper

    <!-- 通常当查询结果是一个集合的话，我们都会在这里定义一个resultMap,用来给下面的方法引用 -->
    <resultMap type="Student" id="StudentResult"><!-- type就是集合中的类型，id随便你怎么起 -->
        <id property="id" column="id"></id><!-- property是类的属性名，对应，column是数据库的字段名 -->
        <result property="name" column="name" />
        <result property="age" column="age" />
        <association property="address" column="addressId" select="com.maya.mappers.AddressMapper.getById"></association>
        <!-- property是属性名，column是外键列，select是对应的根据外键id寻找其表中数据的方法 -->
    </resultMap>

Junit测试结果