mybatis中的一对多

前言：创建项目啥的就不说了，直接开始讲怎么从model开始
我有两个实体类（model）：User和City

一对多的关系是，一个User可以去过很多个City。

City的创建如下

说明，按照表的字段来创建类
package com.tqz.model;

public class City {
    private int city_id;
    private int user_id;
    private String cityname;
    public int getCity_id() {
        return city_id;
    }
    public void setCity_id(int city_id) {
        this.city_id = city_id;
    }
    public int getUser_id() {
        return user_id;
    }
    public void setUser_id(int user_id) {
        this.user_id = user_id;
    }
    public String getCityname() {
        return cityname;
    }
    public void setCityname(String cityname) {
        this.cityname = cityname;
    }
}
User的创建如下：

说明：在User类里面，要写一个private List<City> cities;//指向去过的城市
它是用来存User对应的多个City的
package com.tqz.model;

import java.util.List;

public class User {
    private int id;
    private String username;
    private int tel;
    private String address;
    private List<City> cities;//指向去过的城市
    public List<City> getCities() {
        return cities;
    }
    public void setCities(List<City> cities) {
        this.cities = cities;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public int getTel() {
        return tel;
    }
    public void setTel(int tel) {
        this.tel = tel;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
}
关键的来了：User.xml文件的写法

<?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.tqz.imapper.IUser">

  <resultMap type="User" id="UserBeenToResult">
    <id column="id" jdbcType="INTEGER" property="id"/>
    <result column="username" jdbcType="VARCHAR" property="username"/>
    <result column="tel" jdbcType="INTEGER" property="tel"/>
    <result column="address" jdbcType="VARCHAR" property="address"/>
    <collection property="cities" ofType="City">
        <id column="city_id" jdbcType="INTEGER" property="city_id"/>
        <id column="user_id" jdbcType="INTEGER" property="user_id"/>
        <result column="cityname" jdbcType="VARCHAR" property="cityname"/>
    </collection>
  </resultMap>

  <select id="getUserBeenToCity" resultMap="UserBeenToResult" 
                                                  parameterType="String">
    SELECT u.id,u.username,u.tel,u.address,
    c.city_id,c.user_id,c.cityname
    FROM user u 
    inner join city c
    ON c.user_id=u.id
    WHERE u.username=#{name}<!-- 写好sql语句很关键 -->
  </select>

</mapper>
说明：用resultMap + collection来处理一堆多的关系
用法：在resultMap标签里面再添加collection标签
实现：resultMap的type是User，表示查询最终的结果是User对象（即一个）
collection的ofType是City，表示要整理出的集合是City对象（即多个）
collection的property的值是cities，这个是Uesr的属性
collection下面的内容就是封装对象的操作了
collection操作结束后的结果是一个集合，赋给User的cities

重点：select里面的sql语句
SELECT u.id, u.username, u.tel, u.address,
    c.city_id, c.user_id, c.cityname
FROM user u 
inner join city c
ON c.user_id=u.id
WHERE u.username=#{name}
因为是关联两个表，所以要在查询的字段前面加上字段的标识符【 u.id | c.city_id】
inner join city c表示关联city这个表c是给他起的别名就像u一样
ON c.user_id=u.id
WHERE u.username=#{name}是关联条件
条件：user_id是存在表city里面的
id是存在user里面的
表city里面可以存在多个相同的user_id
c.user_id=u.id就表示在表city里面找与user相同的id（这个id可以有多个，即：一对 多）


测试
public class Demo {
    private static SqlSessionFactory sqlSessionFactory;
    private static Reader reader;
    private static SqlSession session;
    static {
        try {
            reader = Resources.getResourceAsReader("com/tqz/config/Configuration.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public static SqlSessionFactory getSession() {
        return sqlSessionFactory;
    }
        public static void testgetUserBeenToCity(String name) {
        //获取连接
         session = sqlSessionFactory.openSession();
         //获取Mapper
         IUser iUser = session.getMapper(IUser.class);
         User user = new User();
         user = iUser.getUserBeenToCity(name);
         System.out.println("用户id为：" + user.getId());
         System.out.println("用户名为：" + user.getUsername());
         System.out.println("用户tel为：" + user.getTel());
         System.out.println("用户address为：" + user.getAddress());
         System.out.println("*********************************");
         System.out.println("去过的城市：");
         for (City city : user.getCities()) {
             System.out.println(city.getCityname());
         }
     }
    public static void main(String[] args) {
        
        try {
            //一对多：一个人可以去过很多城市
            testgetUserBeenToCity("田钦政");
        } finally {
            session.close();
        }
    }
}
可以看出，查询一个用户去过的城市，结果出来很多个他去过的城市。

源代码放在我的GitHub上 下载