MyBatis之一对多关联查询

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一对多关联

一对多关联和多对一关联的区别：是从一的一端取多的一端，还是从多的一端取一的一端。

表结构（一对多、多对一、一对一的表结构相同）：

-- 用户表
CREATE TABLE `user1`(
    `id` INT PRIMARY KEY AUTO_INCREMENT,
    `user_name` VARCHAR(20),-- 用户姓名
    `address` VARCHAR(60)-- 联系地址
) ENGINE INNODB CHARSET utf8;
INSERT INTO `user1` VALUES(1,'陈大','深圳市南山区');
INSERT INTO `user1` VALUES(2,'王二','深圳市福田区');
INSERT INTO `user1` VALUES(3,'张三','深圳市龙华新区');
INSERT INTO `user1` VALUES(4,'李四','深圳市龙岗区');


-- 卡表
CREATE TABLE `card1`(
    `id` INT PRIMARY KEY AUTO_INCREMENT,
    `card_no` VARCHAR(18),
    `remark` VARCHAR(100),
    `user_id` INT-- 用于关联user1的主键id(不设置外键，避免级联问题)
) ENGINE=INNODB CHARSET=utf8;
INSERT INTO `card1` VALUES(1,'420001','工资卡',1);
INSERT INTO `card1` VALUES(2,'420002','工资卡',2);
INSERT INTO `card1` VALUES(3,'420003','工资卡',3);
INSERT INTO `card1` VALUES(4,'420004','工资卡',3);


-- SELECT * FROM `user1`;
-- SELECT * FROM `card1`;

实体类：Card1

package com.chensan.o2m.entity;

public class Card1 {
	private int id;
	private String cardNo;
	private String remark;
	
	//...省略setter、getter方法
}

实体类：User1

package com.chensan.o2m.entity;

import java.util.List;

public class User1 {
	private int id;
	private String userName;
	private String address;
	private List<Card1> cards;
	
	public String toString(){
		return "[ id = " + id  + ", userName = " 
			+ userName + ", address = " + address + "]";
	}
	//...省略setter、getter方法
}

实体类User1映射文件

<?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.chensan.o2m.entity.User1Mapper">
  <resultMap id="user1" type="com.chensan.o2m.entity.User1">
  	<id property="id" column="user_id"/>
  	<result property="userName" column="user_name"/>
  	<result property="address" column="address"/>
  	
  	<collection property="cards" column="user_id" ofType="com.chensan.o2m.entity.Card1">
  		<id property="id" column="id"/>
  		<result property="cardNo" column="card_no"/>
  		<result property="remark" column="remark"/>
  	</collection>
  </resultMap>
  
  <select id="query" parameterType="int" resultMap="user1">
  	SELECT t1.`id` `user_id`,t1.`user_name`,t1.`address`,t2.`id`,t2.`card_no`,t2.`remark` 
	FROM `user1` t1,`card1` t2 
	WHERE t1.`id`=t2.`user_id` AND t1.`id`=#{id}
  </select>
</mapper>

myBatis配置文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>
  <!-- 和spring整合后 environments配置将废除-->
  <environments default="development">
	<environment id="development">
	  <!-- 使用jdbc事务管理  或者JTA事务管理-->
	  <transactionManager type="JDBC" />
	  <!-- 数据库连接池  第三方组件：c3p0-->
	  <dataSource type="POOLED">
		<property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://localhost:3306/mybatis01"/>
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
	  </dataSource>
	</environment>
  </environments>

  <!-- 加载实体类的映射文件 -->
  <mappers>
    <mapper resource="com/chensan/o2m/mapper/User1Mapper.xml"/>
  </mappers>
</configuration>

测试类

package com.chensan.o2m.test;

import java.io.Reader;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import com.chensan.o2m.entity.Card1;
import com.chensan.o2m.entity.User1;

public class TestO2M {
	private static SqlSessionFactory sqlSessionFactory;
    private static Reader resource;
    
    //创建会话工厂
    static{
        try{
        	resource = Resources.getResourceAsReader("myBatisConfig.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(resource);
        }catch(Exception e){
            e.printStackTrace();
        }
    }
    
    public static SqlSessionFactory getSession(){
        return sqlSessionFactory;
    }
    
    //一对多：查询用户对应卡（银行卡）
    public void getUserCard(){
    	SqlSession sqlSession = sqlSessionFactory.openSession();
    	User1 user = sqlSession.selectOne("com.chensan.o2m.entity.User1Mapper.query", 3);
    	System.out.println(user);
    	for(Card1 card : user.getCards()){
    		System.out.println(
				"[ " + 
				"userId = " + user.getId() + ", " + 
				"userName = " + user.getUserName() + ", " + 
				"address = " + user.getAddress() + ", " + 
				"cardId = " + card.getId() + ", " + 
				"cardNo = " + card.getCardNo() + ", " + 
				"remark = " + card.getRemark() + 
				" ]"
			);
    	}
    	
    	sqlSession.close();
    }

    public static void main(String[] args) {
    	TestO2M testMyBatisOneToMany = new TestO2M();
    	testMyBatisOneToMany.getUserCard();
    }
}

结果：

[ id = 3, userName = 张三, address = 深圳市龙华新区]
[ userId = 3, userName = 张三, address = 深圳市龙华新区, cardId = 3, cardNo = 420003, remark = 工资卡 ]
[ userId = 3, userName = 张三, address = 深圳市龙华新区, cardId = 4, cardNo = 420004, remark = 工资卡 ]

user1和card1的id列同名，如果不对其中一列取别名，则user只关联一个card对象，只有一条记录：

[ id = 3, userName = 张三, address = 深圳市龙华新区]
[ userId = 3, userName = 张三, address = 深圳市龙华新区, cardId = 3, cardNo = 420003, remark = 工资卡 ]
之前很奇怪只有一条记录，参考：http://blog.csdn.net/rain097790/article/details/13615291后才知道是列同名问题。