mybatis 一对多和多对一

在学习MyBatis3的过程中，文档上面一直在强调一个id的东西！在做这个实验的时候，也因为没有理解清楚id含义而导致一对多的“多”中也只有一条数据。id和result的唯一不同是id表示的结果将是当比较对象实例时用到的标识属性。这帮助来改进整体表现，特别是缓存和嵌入结果映射。所以不同数据的id应该唯一区别，不然导致数据结果集只有一条数据。

一、表

二、实体

1.person

package com.kerwin.mybatis.pojo;
import java.util.List;
public class Person {
private int id;
private String name;
private List<Orders> orderList;
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 List<Orders> getOrderList() {
return orderList;
}
public void setOrderList(List<Orders> orderList) {
this.orderList = orderList;
}
@Override
public String toString() {
return "Person [id=" + id + ", name=" + name + "]";
}
public Person() {
super();
// TODO Auto-generated constructor stub
}
public Person(int id, String name, List<Orders> orderList) {
super();
this.id = id;
this.name = name;
this.orderList = orderList;
}
}

2.order

package com.kerwin.mybatis.pojo;
public class Orders {
private int id;
private double price;
private Person person;
public Person getPerson() {
return person;
}
public void setPerson(Person person) {
this.person = person;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
@Override
public String toString() {
return "Orders [id=" + id + ", price=" + price + "]";
}
public Orders() {
super();
// TODO Auto-generated constructor stub
}
}

三、映射mapper文件

1. PersonMapper.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="com.kerwin.mybatis.pojo.Person">
<resultMap type="com.kerwin.mybatis.pojo.Person" id="personreSultMap">
<id column="p_id" property="id"/>
<result column="name" property="name"/>
<collection property="orderList" ofType="com.kerwin.mybatis.pojo.Orders" column="pid">
<id column="o_id" property="id"/>
<result column="price" property="price"/>
</collection>
</resultMap>
<select id="selectPersonFetchOrder" parameterType="int" resultMap="personreSultMap" >
select p.*,o.* from person p,orders o where o.pid=p.p_id and p.p_id=#{id}
</select>
</mapper>

2. OrdersMapper.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="com.kerwin.mybatis.pojo.Orders">
<resultMap type="com.kerwin.mybatis.pojo.Orders" id="OrdersResultMap">
<id column="o_id" property="id"/>
<result column="price" property="price"/>
<association property="person" javaType="com.kerwin.mybatis.pojo.Person">
<id column="p_id" property="id"/>
<result column="name" property="name"/>
</association>
</resultMap>
<select id="selectOrdersFetchPerson" resultMap="OrdersResultMap">
select p.*,o.* from person p,orders o where o.pid=p.p_id and o.o_id=#{id}
</select>
</mapper>

3.sqlMapConfig.xml

<?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="com.kerwin.mybatis.pojo.Author" alias="Author"/>
</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/mybatis"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/kerwin/mybatis/pojo/AuthorMapper.xml"/>
<mapper resource="com/kerwin/mybatis/pojo/PostMapper.xml"/>
<mapper resource="com/kerwin/mybatis/pojo/PersonMapper.xml"/>
<mapper resource="com/kerwin/mybatis/pojo/OrdersMapper.xml"/>
</mappers>
</configuration>

四。测试类

/**
*
*/
package com.kerwin.mybatis.test;
import java.io.InputStream;
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.junit.BeforeClass;
import org.junit.Test;
import com.kerwin.mybatis.pojo.Orders;
import com.kerwin.mybatis.pojo.Person;
/**
* @author Administrator
*
*/
public class PersonAndOrderTest {
private static SqlSessionFactory sessionFactory;
/**
* @throws java.lang.Exception
*/
@BeforeClass
public static void setUpBeforeClass() throws Exception {
SqlSessionFactoryBuilder factoryBuilder = new SqlSessionFactoryBuilder();
InputStream inputStream = Resources.getResourceAsStream("sqlMapConfig.xml");
sessionFactory = factoryBuilder.build(inputStream);
}
//一对多，查询person（一）级联查询订单order（多）
@Test
public void testSelectPersonFetchOrder() throws Exception {
SqlSession session = sessionFactory.openSession();
Person person = session.selectOne("com.kerwin.mybatis.pojo.Person.selectPersonFetchOrder", 1);
System.out.println(person);
System.out.println(person.getOrderList().size());
for(Orders orders : person.getOrderList()){
System.out.println(orders);
}
session.close();
}
//多对一，查询订单order（多）级联查询person（一）
@Test
public void testSelectOrdersFetchPerson() throws Exception{
SqlSession session = sessionFactory.openSession();
Orders orders = session.selectOne("com.kerwin.mybatis.pojo.Orders.selectOrdersFetchPerson", 1);
System.out.println(orders);
System.out.println(orders.getPerson());
session.close();
}
}

五、测试结果

1.一对多，查询person（一）级联查询订单order（多）

2.多对一，查询订单order（多）级联查询person（一）

注意：两张表中的主键id字段名要唯一，例如不能都写id，不然的话，在一对多查询的时候就会出现：级联出来的订单项只有一条记录。我之前就是将两张表的主键id字段名都写为id，导致测试结果级联出来的多一直只有一条数据，具体如下：

巴特西

mybatis 一对多和多对一

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