MyBatis关联映射关系，一对一，一对多，多对多

骑猪看日落 2022-04-17 00:39 415阅读 0赞

**“纵许悠扬度朱户，终愁人影隔窗纱”**

## 前言 ##

客观世界中的对象很少有孤立存在的，例如班级，往往与班级的学生存在关联关系，如果得到某个班级的实例，那么应该可以直接获取班级对应的全部学生。反过来，如果已经得到个学生的实例，那么也应该可以访问该学生对应的班级。这种实例之间的互相访问就是关联关系。  
关联关系是面向对象分析、面向对象设计最重要的知识， My Batis完全可以理解这种关联关系，如果映射得当， My Batis的关联映射将可以大大简化持久层数据的访问。

## 示例 ##

这里只给出映射文件（XML文件）中的关键代码

*  一对一关系  
    例如：一个人对应一个身份证号  
    **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="org.arunner.mapper.PersonMapper">
    
    	<resultMap id="personMapper" type="org.arunner.domain.Person">
    		<id property="id" column="id"/>
    		<result property="name" column="name"/>
    		<result property="age" column="age"/>
    		<result property="sex" column="sex"/>
    		
    		<association property="card" column="card_id" javaType="org.arunner.domain.Card" select="org.arunner.mapper.CardMapper.selectCardById"/>
    	</resultMap>
    
    	
    	<select id="selectPersonById" parameterType="int" resultMap="personMapper">
    		select * from tb_person where id = #{id} ;
    	</select>
    	
    </mapper>

**CardMapper.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="org.arunner.mapper.CardMapper">
    
    
    	
    	<select id="selectCardById" parameterType="int" resultType="org.arunner.domain.Card">
    		select * from tb_card where id = #{id} ;
    	</select>
    
    </mapper>

*  一对多关系  
    例如：一个班级对应多个学生  
    **ClazzMapper.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="org.arunner.mapper.ClazzMapper">
    
    	<resultMap id="clazzResultMap" type="org.arunner.domain.Clazz">
    		<id property="id" column="id"/>
    		<result property="code" column="code"/>
    		<result property="name" column="name"/>
    		
    		<collection property="students" javaType="ArrayList" column="id" ofType="org.arunner.domain.Student"
    					select="org.arunner.mapper.StudentMapper.selectStudentByClazzId" fetchType="lazy">
    			<id property="id" column="id"/>
    			<result property="name" column="name"/>
    			<result property="age" column="age"/>
    			<result property="sex" column="sex"/>
    		</collection>
    	</resultMap>
    
    	
    	<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
    		select * from tb_clazz where id = #{id} ;
    	</select>
    	
    </mapper>

**StudentMapper.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="org.arunner.mapper.StudentMapper">
    
    	<resultMap id="studentResultMap" type="org.arunner.domain.Student">
    		<id property="id" column="id"/>
    		<result property="name" column="name"/>
    		<result property="age" column="age"/>
    		<result property="sex" column="sex"/>
    		
    		<association property="clazz" javaType="org.arunner.domain.Clazz">
    			<id property="id" column="id"/>
    			<result property="code" column="code"/>
    			<result property="name" column="name"/>
    		</association>
    	</resultMap>
    
    	
    	<select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
    		select * from tb_student where clazz_id = #{id} ;
    	</select>
    
    	
    	<select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
    		select * from tb_student s,tb_clazz c where c.id = s.clazz_id and s.id= #{id} ;
    	</select>
    
    </mapper>

*  多对多关系  
    例如：订单、商品和用户的关系  
    **OrderMapper.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="org.arunner.mapper.OrderMapper">
    
    	<resultMap type="org.arunner.domain.Order" id="orderResultMap">
    		<id property="id" column="oid"/>
    	  	<result property="code" column="code"/>
    	  	<result property="total" column="total"/>
    		
    		<association property="user" javaType="org.arunner.domain.User">
    			<id property="id" column="id"/>
    			<result property="username" column="username"/>
    			<result property="loginname" column="loginname"/>
    			<result property="password" column="password"/>
    			<result property="phone" column="phone"/>
    			<result property="address" column="address"/>
    		</association>
    		
    		<collection property="articles" javaType="ArrayList"
    	  column="oid" ofType="org.arunner.domain.Article"
    	  select="org.arunner.mapper.ArticleMapper.selectArticleByOrderId"
    	  fetchType="lazy">
    	  	<id property="id" column="id"/>
    	  	<result property="name" column="name"/>
    	  	<result property="price" column="price"/>
    	  	<result property="remark" column="remark"/>
    	  </collection>
    	</resultMap>
    	
    	
      <select id="selectOrderById" parameterType="int" resultMap="orderResultMap">
      	SELECT u.*,o.id AS oid,CODE,total,user_id
      	 FROM tb_user u,tb_order o
      	WHERE u.id = o.user_id
      	 AND o.id = #{id}
      </select>
      
      
      <select id="selectOrderByUserId" parameterType="int" resultType="org.arunner.domain.Order">
      	SELECT * FROM tb_order WHERE user_id = #{id}
      </select>
      
    </mapper>

**UserMapper.xml**

**ArticleMapper.xml**