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：