orm-mybatis开发系列之二：一对多关联查询

这里介绍mybatis是如何处理一对多的关联查询的。

例如一个用户对应多个订单，他们之间是一对多的关系。在Java实体对象对中，一对多可以根据List和Set来实现，两者在mybatis中都是通过collection标签来配合使用，稍后会做详细配置介绍。

一、创建用户表（User）和订单表（Order），并插入测试数据

-- Create a table named 'user'  CREATE TABLE ym_test.`user` (      `id` int(11) NOT NULL AUTO_INCREMENT,      `name` varchar(50) DEFAULT NULL,      `age` int(11) DEFAULT NULL,      `address` varchar(200) DEFAULT NULL,      PRIMARY KEY (`id`)  ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;    -- Insert a test record  insert into ym_test.`user` VALUES ('1', 'ym', '24', 'guangzhou,tianhe');  DROP TABLE IF EXISTS `order`;  -- Create a table named 'order'  CREATE TABLE ym_test.`order` (      `id` int NOT NULL AUTO_INCREMENT,      `user_id` int NOT NULL,      `title` varchar(100) NOT NULL,      `content` text NOT NULL,      PRIMARY KEY (`id`)  ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;    -- Add several test records  INSERT INTO ym_test.`order`  VALUES  ('1', '1', 'title1', 'content1'),  ('2', '1', 'title2', 'content2'),  ('3', '1', 'title3', 'content3'),  ('4', '1', 'title4', 'content4');  

二、创建表对应的实体类

User.java

package orm.mybatis.model;import java.io.Serializable;import java.util.Set;public class User implements Serializable {private static final long serialVersionUID = 685741321488921919L;private int id;private String name;private int age;private String address;private Set<Order> orders;// 一个用户对应多个订单，一对多关系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 String getAddress() {return address;}public void setAddress(String address) {this.address = address;}public Set<Order> getOrders() {return orders;}public void setOrders(Set<Order> orders) {this.orders = orders;}}

Order.java

package orm.mybatis.model;import java.io.Serializable;public class Order implements Serializable {private static final long serialVersionUID = 5273490273354363601L;private int id;private User user;// 多个订单对应一个用户，多对一关系private String title;private String content;public int getId() {return id;}public void setId(int id) {this.id = id;}public User getUser() {return user;}public void setUser(User user) {this.user = user;}public String getTitle() {return title;}public void setTitle(String title) {this.title = title;}public String getContent() {return content;}public void setContent(String content) {this.content = content;}}

三、配置映射文件
UserMapper.xml

<?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="orm.mybatis.dao.IUserDao"><!-- 配置结果映射  --><resultMap type="User" id="baseResultMap"><result column="id" property="id" /><result column="name" property="name" /><result column="age" property="age" /><result column="address" property="address" /><!-- 一对多配置(ofType指定集合中的对象类型) --><collection property="orders" ofType="Order"><id property="id" column="order_id" javaType="int"  /><result property="title" column="title" javaType="string" /><result property="content" column="content" javaType="string" /></collection></resultMap><select id="getUserById" parameterType="int" resultMap="baseResultMap"><!-- 分别为user的主键id与order的主键id赋值别名，避免因为两个表字段名称相同而注入到对应对象名称冲突 -->select u.id, u.name, u.age, u.address, o.id as order_id, o.title, o.content from ym_test.order o, ym_test.user u where o.user_id=u.id and u.id=#{id}  </select></mapper>

四、编写IUserDao

package orm.mybatis.dao;import orm.mybatis.model.User;public interface IUserDao {User getUserById(int id); }

五、测试方法

package orm.mybatis.main;import java.io.IOException;import java.io.Reader;import java.util.Set;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 org.apache.log4j.Logger;import org.junit.BeforeClass;import org.junit.Test;import orm.mybatis.dao.IUserDao;import orm.mybatis.model.Order;import orm.mybatis.model.User;public class TestMyBatis {private static final Logger log = Logger.getLogger(TestMyBatis.class);private static SqlSessionFactory sqlSessionFactory;private static Reader reader;/** * before和beforeclass区别 * @befor在每个测试方法之前都会运行一次，只需声明成public * @beforeclass在类中只运行一次，必须声明成public static */@BeforeClasspublic static void initial() {try {reader = Resources.getResourceAsReader("mybatis-config.xml");sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);} catch (IOException e) {log.error("Error thrown while reading the configuration: {}", e);} finally {if (reader != null) {try {reader.close();} catch (IOException e) {log.error("Error thrown while closing the reader: {}", e);}}}}@Testpublic void queryTest() {SqlSession sqlSession = sqlSessionFactory.openSession();IUserDao userDao = sqlSession.getMapper(IUserDao.class);User user = userDao.getUserById(1);Set<Order> orders = user.getOrders();if(orders != null && orders.size() > 0) {for(Order o : orders) {System.out.println("id:" + o.getId() + ";title:" + o.getTitle() + ";content:" + o.getContent());}}}}

测试OK