MyBatis学习总结之一对一映射

知识点：JavaType和ofType都是用来指定对象类型的，但是JavaType是用来指定pojo中属性的类型，而ofType指定的是映射到list集合属性中pojo的类型。

本次mybatis的练习是使用的springboot+mybatis

第一步：在mysql 数据库创建students 和cards表并插入数据

create table cards(
	cid int(5) primary key,
	cnum varchar(10)
);

create table students(
	sid int(5) primary key,
	sname varchar(10),
	scid int(5),
	constraint scid_fk foreign key(scid) references cards(cid)
);

insert into cards(cid,cnum) values(1,'111');
insert into students(sid,sname,scid) values(1,'哈哈',1);

select * from cards;
select * from students;

第二步：配置springboot配置文件application.properties并在该文件添加以下配置信息

spring.datasource.url=jdbc:mysql://127.0.0.1:3306/diamond?useUnicode=true&characterEncoding=utf8&autoReconnect=true&allowMultiQueries=true
spring.datasource.username=root
spring.datasource.password=123456
spring.datasource.driver-class-name=com.mysql.jdbc.Driver　　

第三步：创建Domain: Student.java 和Card.java

import lombok.Data;

@Data
public class Card {
    private Integer id;
    private String num;
}

　

@Data
public class Student {
    private Integer id;
    private String name;
    private Card card;
}

第四步：创建StudentCardMapper接口　

import com.longteng.diamond.domain.Student;
import org.apache.ibatis.annotations.Mapper;

@Mapper
public interface StudentCardMapper {
    /**
     * 查询1号学生的信息
     * @param id 表示学生的编号
     */
    Student selectById(Integer id);
}

第五步：创建StudentCardMapper.xml文件

<?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.longteng.diamond.dao.StudentCardMapper">
    <resultMap id="studentCardMap" type="com.longteng.diamond.domain.Student">
        <id property="id" column="sid" jdbcType="INTEGER"></id>
        <result property="name" column="sname" jdbcType="VARCHAR"></result>
        <!-- 引入CardMapper.xml文件中的映射信息
             property表示Student类的关联属性
             resultMap表示引入CardMapper.xml文件的映射类型
        -->
    </resultMap>
    <select id="selectById" parameterType="java.lang.Integer" resultMap="studentCardMap">
      select s.sid,s.sname,s.scid,c.cnum，c.cid
      from students s inner join cards c
      on s.scid= c.cid and s.sid=#{xx}
    </select>

</mapper>

第六步：创建Controller

import com.longteng.diamond.dao.StudentCardMapper;
import com.longteng.diamond.domain.Student;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import javax.annotation.Resource;

@Controller
public class StudentCardController {
    @Resource
    StudentCardMapper studentCardMapper;
    @RequestMapping("/testMapper")
    public @ResponseBody  String test(){
        Student student = studentCardMapper.selectById(1);
        return student.toString();
    }
}

第七步：调用接口http://localhost/testMapper执行结果如下：

Student(id=1, name=哈哈, card=null)

其中card = null 是由于在第五步操作中resultMap 中没有关联card表里面的属性，而在select 查询中却查询了card表的属性值，解决办法有2种

方法1：直接将collection集合元素的属性写为collection的子标签

<?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.longteng.diamond.dao.StudentCardMapper">
    <resultMap id="studentCardMap" type="com.longteng.diamond.domain.Student">
        <id property="id" column="sid" jdbcType="INTEGER"></id>
        <result property="name" column="sname" jdbcType="VARCHAR"></result>
        <result property="cid" column="scid" jdbcType="INTEGER"></result>
        <!-- 引入CardMapper.xml文件中的映射信息
             property表示Student类的关联属性
             resultMap表示引入CardMapper.xml文件的映射类型
        -->
        <collection property="card" ofType="com.longteng.diamond.domain.Card">
            <id property="id" column="cid" ></id>
            <result property="num" column="cnum" jdbcType="VARCHAR"></result>
        </collection>
    </resultMap>

    <select id="selectById" parameterType="java.lang.Integer" resultMap="studentCardMap">
      select s.sid,s.sname,s.scid,c.cnum,c.cid
      from students s inner join cards c
      on s.scid= c.cid and s.sid=#{xx}
    </select>

</mapper> 

调用接口http://localhost/testMapper执行结果如下：

Student(id=1, name=哈哈, card=Card(id=1, num=111))

方法2：通过在collection标签中引用别的mapper的查询方法

第一步创建CardMapper接口

import com.longteng.diamond.domain.Card;
import org.apache.ibatis.annotations.Mapper;

@Mapper
public interface CardMapper {
    Card findCard(Integer id);
}

第二步创建CardMapper.xml文件

<?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.longteng.diamond.dao.CardMapper">
    <resultMap id="cardMapper" type="com.longteng.diamond.domain.Card">
        <id property="id" column="cid" jdbcType="INTEGER"></id>
        <result property="num" column="cnum" jdbcType="VARCHAR"></result>
    </resultMap>
    <select id="findCard"  resultMap="cardMapper">
        SELECT * from cards where cid=#{0};
    </select>
</mapper>

第三步在StudentCardMapper.xml文件里面引入CardMapper.xml文件

<?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.longteng.diamond.dao.StudentCardMapper">
    <resultMap id="studentCardMap" type="com.longteng.diamond.domain.Student">
        <id property="id" column="sid" jdbcType="INTEGER"></id>
        <result property="name" column="sname" jdbcType="VARCHAR"></result>
        <result property="cid" column="scid" jdbcType="INTEGER"></result>
           <!-- 方法2：
            引入CardMapper.xml文件中的映射信息
             property表示Student类的关联属性

        -->
        <collection property="card" column="scid" jdbcType="INTEGER" select="com.longteng.diamond.dao.CardMapper.findCard"/>
    </resultMap>

    <select id="selectById" parameterType="java.lang.Integer" resultMap="studentCardMap">
      select s.sid,s.sname,s.scid,c.cnum,c.cid
      from students s inner join cards c
      on s.scid= c.cid and s.sid=#{xx}
    </select>

</mapper>