MyBatis多表操作情形二：一对多

电玩女神 2022-05-12 02:58 150阅读 0赞

*  例子：一个人可以有多条狗，所以person表和dog表，就有一对多的关系。在多的一方外键关联少的一方，也就是dog表里的pid关联person表的id，如下：张三有两条狗，旺财和旺旺。那么mybatis里是如何将这种关系映射到对象上的呢？
 *  数据库表person和dog  
    ![这里写图片描述][70]  
    ![这里写图片描述][70 1]

package com.yicha.entity;
    //Person2.java
    import java.util.List;
    
    public class Person2 { 
        private int id;
        private String name;
        private List<Dog> dogList;
        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<Dog> getDogList() {
            return dogList;
        }
        public void setDogList(List<Dog> dogList) {
            this.dogList = dogList;
        }
    }

package com.yicha.entity;
    //Dog.java
    public class Dog {
        private int id;
        private String name;
        private int pid;
        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 int getPid() {
            return pid;
        }
        public void setPid(int pid) {
            this.pid = pid;
        }
    }

Person2.xml

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//ibatis.apache.org//DTD SQL Map 3.0//EN" "http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">
    <mapper namespace="com.yicha.entity.Person2">
        
        <resultMap type="Person2" id="myMap">
            <result property="id" column="id"/>
            <result property="name" column="pname"/>
            <collection property="dogList" ofType="Dog">
                <result property="name" column="dname"/>
            </collection>   
        </resultMap>
        <select id="find1" resultMap="myMap">
            SELECT a.id,a.name AS pname,b.name AS dname
            FROM person a
            LEFT JOIN dog b
            ON a.id=b.pid 
            WHERE a.id=1    
        </select>
    </mapper>

SqlMapConfig.xml

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE configuration PUBLIC "-//ibatis.apache.org//DTD SQL Map Config 3.0//EN" "http://ibatis.apache.org/dtd/ibatis-3-config.dtd">
    <configuration>
        
        <settings>
             <setting name="lazyLoadingEnabled" value="true"/>  
             <setting name="aggressiveLazyLoading" value="false"/>  
        </settings>
        
        <typeAliases>
            <typeAlias type="com.yicha.entity.Person" alias="Person"/>
            <typeAlias type="com.yicha.entity.Card" alias="Card"/>
            <typeAlias type="com.yicha.entity.Dog" alias="Dog"/>
            <typeAlias type="com.yicha.entity.Person2" alias="Person2"/>
        </typeAliases>
    
        
        <environments default="e1">
            <environment id="e1">
                
                <transactionManager type="JDBC"></transactionManager>
                
                <dataSource type="POOLED">
                    <property name="driver" value="com.mysql.jdbc.Driver"/>
                    <property name="url" value="jdbc:mysql://localhost:3306/test"/>
                    <property name="username" value="root"/>
                    <property name="password" value="root1205"/>
                </dataSource>
            </environment>
        </environments>
    
        
        <mappers>
            <mapper resource="com/yicha/entity/Person.xml"/>
            <mapper resource="com/yicha/entity/Card.xml"/>
            <mapper resource="com/yicha/entity/Person2.xml"/>
        </mappers>
    </configuration>

测试类

package com.yicha.test;
    //Demo1.java
    import java.io.IOException;
    import java.io.Reader;
    
    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 com.yicha.entity.Person;
    
    public class Demo1 {
    
        public static void main(String[] args) {
            SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
            SqlSessionFactory factory = null;
            SqlSession session = null;
            try {
                Reader reader = Resources.getResourceAsReader("SqlMapConfig.xml");
                factory = builder.build(reader);
                session = factory.openSession();
            } catch (IOException e) {
                e.printStackTrace();
            }
    //      Person p = session.selectOne("find1"); 
    //      System.out.println(p.getCard().getCardNum());
            session.selectOne("com.yicha.entity.Person2.find1");
            //注意这里仍然用的是selectOne，实际上查询结果是两条数据，但是封装到一个Person2对象里了
            //这种封装避免了冗余封装
        }
    }

结果：  
![这里写图片描述][70 2]

[70]: /images/20220512/189e0051f72b4d15a8f69c0e388d667f.png
[70 1]: /images/20220512/77449151cbe34b038cba90a2373f11ef.png
[70 2]: /images/20220512/493d06a77fe848f9bbc3f53e657935bd.png