Mybatis入门---一对多、多对多

前几天自己配置了Mybatis的高级查询：一对多和多对多，现在记录一下，方便以后用到的时候再回顾，下面是具体的操作步骤

一、首先就是配置Mybatis的xml文件及mapper的xml文件，在这里就不多说了，之前写过这个基本的配置，可以参考：https://www.cnblogs.com/blogs-of-xiu/p/10405407.html

经过配置完基本的xml文件之后，我们就开始实现一对多和多对多的需求。

我这里举的例子是用户和订单，一个用户可以对应多个订单，所以需要的表也是用户表和订单表。数据结构如下：之前的文章中是一对一查询，只用到了user表，现在多了三个表分别是orders【订单表】、order_detail【订单详情】、product【商品表】。

用户与订单的关系为一对多，订单与订单明细的关系是一对多，订单明细与商品表的关系是一对一，用户与商品的关系是多对多。

在这里我们使用的关系是用户与订单【一对多】和用户与商品的关系【多对多】，下面会说明多对多的关系如何配置。

整个数据库的sql结构如下【从逻辑上面推敲，可能有些字段设置的不合适】：

/*
Navicat MySQL Data TransferSource Server         : xxx
Source Server Version : 50540
Source Host           : localhost:3306
Source Database       : mybatisTarget Server Type    : MYSQL
Target Server Version : 50540
File Encoding         : 65001Date: 2019-03-14 11:07:30
*/SET FOREIGN_KEY_CHECKS=0;-- ----------------------------
-- Table structure for orders
-- ----------------------------
DROP TABLE IF EXISTS `orders`;
CREATE TABLE `orders` (`id` int(11) NOT NULL AUTO_INCREMENT,`user_id` int(11) DEFAULT NULL,`order_number` int(11) DEFAULT NULL,PRIMARY KEY (`id`),KEY `user_id` (`user_id`),CONSTRAINT `user_id` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;-- ----------------------------
-- Table structure for order_detail
-- ----------------------------
DROP TABLE IF EXISTS `order_detail`;
CREATE TABLE `order_detail` (`id` int(11) NOT NULL AUTO_INCREMENT,`order_id` int(11) NOT NULL,`product_id` int(255) DEFAULT NULL,PRIMARY KEY (`id`),KEY `order_id` (`order_id`),KEY `product_id` (`product_id`),CONSTRAINT `order_id` FOREIGN KEY (`order_id`) REFERENCES `orders` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,CONSTRAINT `product_id` FOREIGN KEY (`product_id`) REFERENCES `product` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8;-- ----------------------------
-- Table structure for product
-- ----------------------------
DROP TABLE IF EXISTS `product`;
CREATE TABLE `product` (`id` int(11) NOT NULL AUTO_INCREMENT,`product_name` varchar(255) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;-- ----------------------------
-- Table structure for user
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (`id` int(11) NOT NULL AUTO_INCREMENT,`username` varchar(255) DEFAULT NULL,`password` varchar(255) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

二、经过数据库的设计之后，我们开始进行实现代码，下面是主要的类：

1、实体类中除了product类不需要加入其它类的对象之外，另外几个类中都需要加入其它实体类的对象。

【原因：例如用户 UserBean 中，属性需要加上订单 Orders 的对象，因为订单对于用户来说是多的一方】

具体的代码如下：

UserBean：

package bean;import java.util.List;public class UserBean {private int id;private String username;private String password;private List<Orders> ordersList;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 String getPassword() {return password;}public void setPassword(String password) {this.password = password;}public List<Orders> getOrdersList() {return ordersList;}public void setOrdersList(List<Orders> ordersList) {this.ordersList = ordersList;}@Overridepublic String toString() {return "UserBean [id=" + id + ", username=" + username + ", password=" + password + ", ordersList=" + ordersList+ "]";}public UserBean(String username, String password) {super();this.username = username;this.password = password;}public UserBean(int id, String username, String password, List<Orders> ordersList) {super();this.id = id;this.username = username;this.password = password;this.ordersList = ordersList;}public UserBean() {super();}}

订单类  Orders：

package bean;import java.util.List;public class Orders {private int id;private int user_id;private String number;private List<Order_Detail> order_detail;public int getId() {return id;}public void setId(int id) {this.id = id;}public int getUser_id() {return user_id;}public void setUser_id(int user_id) {this.user_id = user_id;}public String getNumber() {return number;}public void setNumber(String number) {this.number = number;}public List<Order_Detail> getOrder_detail() {return order_detail;}public void setOrder_detail(List<Order_Detail> order_detail) {this.order_detail = order_detail;}@Overridepublic String toString() {return "Orders [id=" + id + ", user_id=" + user_id + ", number=" + number + ", order_detail=" + order_detail+ "]";}public Orders(int id, int user_id, String number, List<Order_Detail> order_detail) {super();this.id = id;this.user_id = user_id;this.number = number;this.order_detail = order_detail;}public Orders() {super();}}

订单明细 Order_Detail：

package bean;public class Order_Detail {private int id;private int order_id;private int product_id;private Product product;public int getId() {return id;}public void setId(int id) {this.id = id;}public int getOrder_id() {return order_id;}public void setOrder_id(int order_id) {this.order_id = order_id;}public int getProduct_id() {return product_id;}public void setProduct_id(int product_id) {this.product_id = product_id;}public Product getProduct() {return product;}public void setProduct(Product product) {this.product = product;}@Overridepublic String toString() {return "Order_Detail [id=" + id + ", order_id=" + order_id + ", product_id=" + product_id + ", product="+ product + "]";}public Order_Detail(int id, int order_id, int product_id, Product product) {super();this.id = id;this.order_id = order_id;this.product_id = product_id;this.product = product;}public Order_Detail() {super();}
}

商品类  Product：

package bean;public class Product {private int id;private String product_name;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getProduct_name() {return product_name;}public void setProduct_name(String product_name) {this.product_name = product_name;}@Overridepublic String toString() {return "Product [id=" + id + ", product_name=" + product_name + "]";}public Product(int id, String product_name) {super();this.id = id;this.product_name = product_name;}public Product() {super();}}

2、mapper 接口和mapper.xml文件：

UserMapper 接口：

package mapper;import java.util.List;import bean.Orders;
import bean.UserBean;public interface UserMapper {/*** description select all users*/public List<UserBean> selectAllUser();/*** description insert*/public int insertUser(UserBean userbean);/*** description delete user by id* */public int deleteUserById(int id);/*** description update user* */public int updateUser(UserBean userbean);/*** description select user by id* */public UserBean selectUserById(int id);// 一对多 查询一个用户对应的多个订单public List<Orders> selectOrdersOfUser();// 多对多 查询一个用户对应的多个商品public List<Orders> selectProductOfUser();}

xml文件 UserMapper.xml：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org/DTD Mapper 3.0" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="mapper.UserMapper"><resultMap id="usermap" type="UserBean"><id property="id" column="id" javaType="java.lang.Integer" /><result property="username" column="username" javaType="java.lang.String" /><result property="password" column="password" javaType="java.lang.String" /></resultMap><!-- 一对多 查询用户的订单 --><resultMap id="ordersmap" type="UserBean"><!--这个id的column是你要映射到SQL语句中的，这个property是你从真实的beans实体类的属性中的id --><id column="id" property="id" /><result column="username" property="username" /><result column="password" property="password" /><!--因为这个地方的是一对多，要关联到的是一个集合所以使用collection --><collection property="ordersList" ofType="Orders"><id column="id" property="id" /><result column="order_number" property="number" /></collection></resultMap><resultMap id="selectProductOfUserMap" type="UserBean"><id column="id" property="id" /><result column="username" property="username" /><result column="password" property="password" /><!-- 通过用户查询订单  ordersList  字段来自UserBean实体类中声明的Order对象名，以下的情况都是--> 
　　　　<collection property="ordersList" ofType="Orders">

　　　　<id column="id" property="id" /> 
<result column="order_number" property="number" /> 
<!-- 通过订单查询订单明细 --> <collection property="order_detail" ofType="Order_Detail"> 
<id column="id" property="id" /> 
<result column="order_id" property="order_id" />
 <result column="product_id" property="product_id" /> 
<!-- 通过订单明细查询商品 --> <association property="product" javaType="Product"> 
<id column="id" property="id" /> <result column="product_name" property="product_name" />
 </association> </collection> </collection> </resultMap> 
<select id="selectAllUser" resultMap="usermap"> 
select * from user </select> <insert id="insertUser"> 
insert into user(username,password)values(#{username},#{password}) </insert> 
<delete id="deleteUserById"> delete from user where id=#{id} </delete>
 <update id="updateUser"> update user set username=#{username},password=#{password} where id=#{id} </update>
 <select id="selectUserById" resultMap="usermap"> select * from user where id=#{id} </select>
 <!-- 一对多 查询用户的订单 --> 
<select id="selectOrdersOfUser" resultMap="ordersmap"> select t1.*, t2.order_number from user t1, orders t2 where t1.id=t2.user_id </select> 
<select id="selectProductOfUser" resultMap="selectProductOfUserMap">

 select t1.id as user_id,t1.username, t2.id as order_id,t2.order_number, t3.id as order_detail_id,t3.product_id, t4.id as product_id,t4.product_name 
from user t1, orders t2, order_detail t3, product t4 where t1.id=t2.user_id and t3.order_id=t2.id and t3.product_id=t4.id </select> </mapper>

3、逻辑层和测试 我写在了service类，但是这样不太符合规范

UserService类：

package service;import java.util.List;import org.apache.ibatis.session.SqlSession;import bean.UserBean;
import mapper.UserMapper;
import tools.DB;public class UserService {public static void main(String[] args) {// selectAllUser();// insertUser();// deleteUserById();// updateUser();// 一对多 查询用户的订单// selectOrdersOfUser();// 多对多 查询用户对应的商品
        selectProductOfUser();}private static void selectAllUser() {SqlSession session = DB.getSession();UserMapper mapper = session.getMapper(UserMapper.class);try {List<UserBean> user = mapper.selectAllUser();System.out.println(user.toString());session.commit();} catch (Exception e) {e.printStackTrace();session.rollback();}}private void insertUser() {SqlSession session = DB.getSession();UserMapper mapper = session.getMapper(UserMapper.class);try {UserBean userbean = new UserBean("zs", "123");mapper.insertUser(userbean);System.out.println(userbean.toString());session.commit();} catch (Exception e) {e.printStackTrace();session.rollback();}}private static void deleteUserById() {SqlSession session = DB.getSession();UserMapper mapper = session.getMapper(UserMapper.class);try {mapper.deleteUserById(3);System.out.println("删除成功");session.commit();} catch (Exception e) {e.printStackTrace();session.rollback();}}private static void updateUser() {SqlSession session = DB.getSession();UserMapper mapper = session.getMapper(UserMapper.class);UserBean userbean = mapper.selectUserById(4);System.out.println("修改之前的userbean：" + userbean);try {userbean.setUsername("hhh");userbean.setPassword("111");mapper.updateUser(userbean);System.out.println("修改之后的userbean：" + userbean);session.commit();} catch (Exception e) {e.printStackTrace();session.rollback();}}// 一对多 查询用户订单private static void selectOrdersOfUser() {SqlSession session = DB.getSession();UserMapper mapper = session.getMapper(UserMapper.class);System.out.println(mapper.selectOrdersOfUser().toString());}// 多对多 查询用户对应的商品private static void selectProductOfUser() {SqlSession session = DB.getSession();UserMapper mapper = session.getMapper(UserMapper.class);System.out.println(mapper.selectProductOfUser().toString());}
}

4、mybatis.xml 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><typeAliases><typeAlias type="bean.UserBean" alias="UserBean"/><typeAlias type="bean.Orders" alias="Orders"/><typeAlias type="bean.Order_Detail" alias="Order_Detail"/><typeAlias type="bean.Product" alias="Product"/></typeAliases><environments default="cybatis"><environment id="cybatis"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="com.mysql.jdbc.Driver"/><property name="url" value="jdbc:mysql://localhost:3306/mybatis"/><property name="username" value="root"/><property name="password" value="123456"/></dataSource></environment></environments><mappers><package name="mapper"/></mappers>
</configuration>

5、一对多的关键点是collection标签的配置，多对多中的关键点事collection和association标签的配置。collection里面property的值是来自一对多中多的那方的对象。

多对多的关系中，我们需要将多对多关系拆分成一对多来实现。

【例如多个用户对应多个商品，可以拆分成 ：

一个用户对应多个订单（一对多）+ 一个订单对应多个订单明细（一对多） + 一个订单明细对应一个商品（一对一），最终实现用户->商品的联系。】

6、以上就是主要的代码实现，但是这样正常运行的话，在控制台出现的结果和我们将sql语句粘贴到数据库中运行得到的结果不一致。不一致的现象就是在数据库中正常得到我们需要的数据，但是在控制台中得出的却是相同用户id多个订单的情况，只能查到一条数据，原因是mybatis查到主键为id的数据，只会查到第一条。解决办法就是将表中的主键为id的修改一下，不让主键名为id，这样查到的结果就是我们得到的全部数据了。