445、Java框架99 -【MyBatis - 多对多】 2020.12.23

0、定义多对多关系

1、表结构

2、导入数据

3、实体类

4、映射文件

5、修改mybatis-config.xml

6、查询操作

7、建立关系

8、删除关系

9、修改关系

10、参考链接

0、定义多对多关系

本知识点是基于多对一的基础上进行。
在学习之前首先要理清楚多对多的关系，这里以订单Order和产品Product为例：
一张订单里可以包含多种产品
一种产品可以出现在多张订单里
这就是多对多关系
为了维系多对多关系，必须要一个中间表。在这里我们使用订单项(OrderItem)表来作为中间表

本知识点讲解如何查询多对多关系，建立多对多关系，删除多对多关系。

1、表结构

create table order_ (id int(11) NOT NULL AUTO_INCREMENT,code varchar(32) DEFAULT NULL,PRIMARY KEY (id)
) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;create table order_item_(id int(11) NOT NULL AUTO_INCREMENT, oid int ,pid int ,number int ,PRIMARY KEY(id)
)AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

2、导入数据

在一对多关系中准备的数据里已经有Product数据里，这里就只准备订单数据和订单项数据：
1. 插入两个订单
2. 插入6条订单项数据，建立如下关系
2.1 订单1对应产品 1，2，3
2.2 订单2对应产品 2，3，4

INSERT INTO order_ VALUES (1,'code000A');
INSERT INTO order_ VALUES (2,'code000B');INSERT INTO order_item_ VALUES (null, 1, 1, 100);
INSERT INTO order_item_ VALUES (null, 1, 2, 100);
INSERT INTO order_item_ VALUES (null, 1, 3, 100);
INSERT INTO order_item_ VALUES (null, 2, 2, 100);
INSERT INTO order_item_ VALUES (null, 2, 3, 100);
INSERT INTO order_item_ VALUES (null, 2, 4, 100);

3、实体类

实体类Order和OrderItem

OrderItem.java

package com.how2java.pojo;public class OrderItem {private int id;private int number;private Order order;private Product product;public int getId() {return id;}public void setId(int id) {this.id = id;}public int getNumber() {return number;}public void setNumber(int number) {this.number = number;}public Order getOrder() {return order;}public void setOrder(Order order) {this.order = order;}public Product getProduct() {return product;}public void setProduct(Product product) {this.product = product;}}

Order.java

package com.how2java.pojo;import java.util.List;public class Order {private int id;private String code;List<OrderItem> orderItems;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getCode() {return code;}public void setCode(String code) {this.code = code;}public List<OrderItem> getOrderItems() {return orderItems;}public void setOrderItems(List<OrderItem> orderItems) {this.orderItems = orderItems;}}

4、映射文件

关于映射文件的sql语句说明在后续测试的时候会讲解

Order.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.how2java.pojo"><resultMap type="Order" id="orderBean"><id column="oid" property="id" /><result column="code" property="code" /><collection property="orderItems" ofType="OrderItem"><id column="oiid" property="id" /><result column="number" property="number" /><association property="product" javaType="Product"><id column="pid" property="id"/><result column="pname" property="name"/><result column="price" property="price"/></association>               </collection></resultMap><select id="listOrder" resultMap="orderBean">select o.*,p.*,oi.*, o.id 'oid', p.id 'pid', oi.id 'oiid', p.name 'pname'from order_ oleft join order_item_ oi    on o.id =oi.oidleft join product_ p on p.id = oi.pid</select><select id="getOrder" resultMap="orderBean">select o.*,p.*,oi.*, o.id 'oid', p.id 'pid', oi.id 'oiid', p.name 'pname'from order_ oleft join order_item_ oi on o.id =oi.oidleft join product_ p on p.id = oi.pidwhere o.id = #{id}</select></mapper>

Product.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.how2java.pojo"><resultMap type="Product" id="productBean"><id column="pid" property="id" /><result column="pname" property="name" /><result column="price" property="price" /><!-- 多对一的关系 --><!-- property: 指的是属性名称, javaType：指的是属性的类型 --><association property="category" javaType="Category"><id column="cid" property="id"/><result column="cname" property="name"/></association></resultMap><select id="listProduct" resultMap="productBean">select c.*, p.*, c.id 'cid', p.id 'pid', c.name 'cname', p.name 'pname'from category_ cleft join product_ p on c.id = p.cid</select>   <select id="getProduct" resultMap="productBean">select c.*, p.*, c.id 'cid', p.id 'pid', c.name 'cname', p.name 'pname'from category_ cleft join product_ p on c.id = p.cidwhere p.id = #{id}</select>   </mapper>

OrderItem.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.how2java.pojo"><insert id="addOrderItem" parameterType="OrderItem">insert into order_item_values(null,#{order.id},#{product.id},#{number})</insert>   <insert id="deleteOrderItem" parameterType="OrderItem">delete from order_item_where oid = #{order.id} and pid = #{product.id}</insert>   </mapper>

5、修改mybatis-config.xml

添加对于Order.xml和OrderItem的映射

<?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><package name="com.how2java.pojo"/></typeAliases><environments default="development"><environment id="development"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="com.mysql.jdbc.Driver"/><property name="url" value="jdbc:mysql://localhost:3306/how2java?characterEncoding=UTF-8"/><property name="username" value="root"/><property name="password" value="admin"/></dataSource></environment></environments><mappers><mapper resource="com/how2java/pojo/Category.xml"/><mapper resource="com/how2java/pojo/Product.xml"/><mapper resource="com/how2java/pojo/Order.xml"/><mapper resource="com/how2java/pojo/OrderItem.xml"/></mappers>
</configuration>

6、查询操作

如图所示，查询出所有的订单，然后遍历每个订单下的多条订单项，以及订单项对应的产品名称，价格，购买数量

通过Order.xml的listOrder对应的sql语句进行查询：

<select id="listOrder" resultMap="orderBean">select o.*,p.*,oi.*, o.id 'oid', p.id 'pid', oi.id 'oiid', p.name 'pname' from order_ o left join order_item_ oi    on o.id =oi.oid left join product_ p on p.id = oi.pid
</select>

联合order_, order_item_, product_ 三张表进行查询

<resultMap type="Order" id="orderBean"><id column="oid" property="id" /><result column="code" property="code" /><collection property="orderItems" ofType="OrderItem"><id column="oiid" property="id" /><result column="number" property="number" /><association property="product" javaType="Product"><id column="pid" property="id"/><result column="pname" property="name"/><result column="price" property="price"/></association>              </collection>
</resultMap>

查询结果 id和code字段放在Order对象里，然后通过一对多的<collection>标签把oiid和number放在OrderItem对象里，最后把pid,pname,price放进Product对象里。

package com.how2java;import java.io.IOException;
import java.io.InputStream;
import java.util.List;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.how2java.pojo.Order;
import com.how2java.pojo.OrderItem;
import com.how2java.pojo.Product;public class TestMybatis {public static void main(String[] args) throws IOException {String resource = "mybatis-config.xml";InputStream inputStream = Resources.getResourceAsStream(resource);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession session = sqlSessionFactory.openSession();listOrder(session);session.commit();session.close();}private static void listOrder(SqlSession session) {List<Order> os = session.selectList("listOrder");for (Order o : os) {System.out.println(o.getCode());List<OrderItem> ois= o.getOrderItems();for (OrderItem oi : ois) {System.out.format("\t%s\t%f\t%d%n", oi.getProduct().getName(),oi.getProduct().getPrice(),oi.getNumber());}}}
}

7、建立关系

如图所示，建立了让订单000A和产品z建立了关系
首先通过id分别获取Order对象和Product对象，然后创建一个新的OrderItem对象，接着设置Order，设置Product，设置数量，最后调用"addOrderItem" 对应的sql语句插入数据。

Order o1 = session.selectOne("getOrder", 1);
Product p6 = session.selectOne("getProduct", 6);
OrderItem oi = new OrderItem();
oi.setProduct(p6);
oi.setOrder(o1);
oi.setNumber(200);
session.insert("addOrderItem", oi);

addOrderItem调用insert into 语句插入一条OrderItem记录

<insert id="addOrderItem" parameterType="OrderItem">insert into order_item_ values(null,#{order.id},#{product.id},#{number})
</insert>

package com.how2java;import java.io.IOException;
import java.io.InputStream;
import java.util.List;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.how2java.pojo.Order;
import com.how2java.pojo.OrderItem;
import com.how2java.pojo.Product;public class TestMybatis {public static void main(String[] args) throws IOException {String resource = "mybatis-config.xml";InputStream inputStream = Resources.getResourceAsStream(resource);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession session = sqlSessionFactory.openSession();addOrderItem(session);listOrder(session);session.commit();session.close();}private static void addOrderItem(SqlSession session) {Order o1 = session.selectOne("getOrder", 1);Product p6 = session.selectOne("getProduct", 6);OrderItem oi = new OrderItem();oi.setProduct(p6);oi.setOrder(o1);oi.setNumber(200);session.insert("addOrderItem", oi);}private static void listOrder(SqlSession session) {List<Order> os = session.selectList("listOrder");for (Order o : os) {System.out.println(o.getCode());List<OrderItem> ois = o.getOrderItems();for (OrderItem oi : ois) {System.out.format("\t%s\t%f\t%d%n", oi.getProduct().getName(), oi.getProduct().getPrice(),oi.getNumber());}}}
}

8、删除关系

如图所示，删除了订单00A和产品z的关系，再次查询，就看不到产品z了。
删除关系的时候，通过订单id(1)和产品id(6)进行删除。
其实所谓的删除关系，就是删除掉OrderItem记录。

package com.how2java;import java.io.IOException;
import java.io.InputStream;
import java.util.List;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.how2java.pojo.Order;
import com.how2java.pojo.OrderItem;
import com.how2java.pojo.Product;public class TestMybatis {public static void main(String[] args) throws IOException {String resource = "mybatis-config.xml";InputStream inputStream = Resources.getResourceAsStream(resource);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession session = sqlSessionFactory.openSession();deleteOrderItem(session);
//        addOrderItem(session);listOrder(session);session.commit();session.close();}private static void deleteOrderItem(SqlSession session) {Order o1 = session.selectOne("getOrder",1);Product p6 = session.selectOne("getProduct",6);OrderItem oi = new OrderItem();oi.setProduct(p6);oi.setOrder(o1);session.delete("deleteOrderItem", oi);     }private static void addOrderItem(SqlSession session) {Order o1 = session.selectOne("getOrder",1);Product p6 = session.selectOne("getProduct",6);OrderItem oi = new OrderItem();oi.setProduct(p6);oi.setOrder(o1);oi.setNumber(200);session.insert("addOrderItem", oi);}private static void listOrder(SqlSession session) {List<Order> os = session.selectList("listOrder");for (Order o : os) {System.out.println(o.getCode());List<OrderItem> ois= o.getOrderItems();for (OrderItem oi : ois) {System.out.format("\t%s\t%f\t%d%n", oi.getProduct().getName(),oi.getProduct().getPrice(),oi.getNumber());}}}
}

9、修改关系

多对多不存在修改关系的做法，就是删除旧的，然后新增一条即达到修改的效果。

10、参考链接

[01] How2j - MyBatis - 多对多