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：