MyBatis一对多和多对一-优快云博客

本文详细解析了在MyBatis3中实现一对多关系时，如何正确设置id与result的区别，避免出现数据结果集只有一条数据的问题。通过创建表、实体类、映射文件及测试类，展示了正确的配置方法和测试结果。

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;  
        }  
      
    }

1.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）