接续上一节：MyBatis之多对一查询。
本节目的是实现多对多关系。
什么是多对多关系呢？
一个人在淘宝下单可以包括铅笔，书，耳机....... （一个Order包含多个不同的Product）， 但是铅笔这个产品也可以出现在很多人的订单里（一个Product可以属于多个不同的Order），这就是多对多关系。这里我们使用订单项(OrderItem)作为中间表来实现多对多关系，Order中有多个OrderItem，一个OrderItem包含某一种Product及其数量。
新增实体类Order和OrderItem
package com.nyf.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;
}
}
package com.nyf.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;
}
}
在com.nyf.config包下新增Order.xml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.nyf.config">
<!-- 查出来的订单，会包括多个OrderItem，故使用了collection标签。
在一个订单项中，Product有number个但是仍属于同一种(以"种类"为单位)，
所以，在类型为OrderItem的collection内部使用了association标签。
-->
<resultMap type="Order" id="orderBean">
<id column="oid" property="id" />
<result column="code" property="code" />
<!-- collection标签体现Order和orderItem一对多 -->
<collection property="orderItems" ofType="OrderItem">
<id column="oiid" property="id" />
<result column="number" property="number" />
<!-- association体现了多对一 -->
<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_ o
left join order_item_ oi on o.id =oi.oid
left 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_ o
left join order_item_ oi on o.id =oi.oid
left join product_ p on p.id = oi.pid
where o.id = #{id}
</select>
</mapper>
在com.nyf.config包下新增OrderItem.xml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.nyf.config">
<!-- null的位置是ID，建表的时候设置的自增 -->
<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>
修改Product.xml文件，添加getProduct方法。如下所示：
<?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.nyf.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>
<!-- 根据id查询Product, 关联将Orders查询出来 -->
<select id="listProduct" resultMap="productBean">
select c.id ‘cid‘, p.id ‘pid‘, c.name ‘cname‘, p.name ‘pname‘ from category_ c left join product_ p on c.id = p.cid
</select>
<select id="getProduct" resultMap="productBean">
select c.id ‘cid‘, p.id ‘pid‘, c.name ‘cname‘, p.name ‘pname‘
from category_ c
left join product_ p on c.id = p.cid
where p.id = #{id}
</select>
</mapper>
修改mybatis-config.xml，typeAliases标签内声明Order和OrderItem的别名，mappers标签内添加对于Order.xml和OrderItem的映射，如下所示：
<?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用来给com.nyf.pojo.Category取别名为Category -->
<typeAliases>
<typeAlias alias="Category" type="com.nyf.pojo.Category" />
<typeAlias alias="Product" type="com.nyf.pojo.Product" />
<typeAlias alias="Order" type="com.nyf.pojo.Order" />
<typeAlias alias="OrderItem" type="com.nyf.pojo.OrderItem" />
</typeAliases>
<!-- environments配置要连接的数据库 -->
<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/nyf?characterEncoding=UTF-8"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<!-- mappers,指明实体类对应的配置文件 -->
<mappers>
<mapper resource="com/nyf/config/Category.xml"/>
<mapper resource="com/nyf/config/Product.xml"/>
<mapper resource="com/nyf/config/Order.xml"/>
<mapper resource="com/nyf/config/OrderItem.xml"/>
</mappers>
</configuration>
package com.nyf.tests;
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.nyf.pojo.Order;
import com.nyf.pojo.OrderItem;
import com.nyf.pojo.Product;
public class ManyToMany {
public static void main(String[] args) throws IOException {
String resource = "com/nyf/config/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = sqlSessionFactory.openSession();
addOrderItem(session);
listOrder(session);
deleteOrderItem(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);
}
/*
* 删除，就是删除OrderItem这个中间项，让Order和Product解除关系
*/
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);
}
/*
* 添加，就是添加一个OrderItem中间项，让Order和Product建立关系
*/
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());
}
}
}
}
正确结果应该如下所示：
code000A
product a 88.879997 100
product b 88.879997 100
product c 88.879997 100
product z 88.879997 200
code000B
product b 88.879997 100
product c 88.879997 100
product x 88.879997 100
code000A
product a 88.879997 100
product b 88.879997 100
product c 88.879997 100
code000B
product b 88.879997 100
product c 88.879997 100
product x 88.879997 100
原文：https://www.cnblogs.com/nyfblog/p/13861619.html