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

order.jpg

person.jpg

二、实体
1.person

[java] view plain copy

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;  
}

}

[java] view plain copy

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

[java] view plain copy

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  
}

}

[java] view plain copy

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

[html] view plain copy

<?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">

</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>

[html] view plain copy

<?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">

</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

[html] view plain copy

<?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">

<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>

[html] view plain copy

<?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">

<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

[html] view plain copy

<?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>

[html] view plain copy

<?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>

四。测试类

[java] view plain copy

/**

*/
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();  
}

}

[java] view plain copy

/**

*/
package com.kerwin.mybatis.test;

import java.io.InputStream;

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）

一对多.jpg

2.多对一，查询订单order（多）级联查询person（一）
**

多对一.jpg

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：
**

ps.jpg

MyBatis一对多和多对一

推荐阅读更多精彩内容