【Mybatis】（八）高级映射关系（一对一、一对多、多对多）-阿里云开发者社区

【Mybatis】（八）高级映射关系（一对一、一对多、多对多）

2022-05-30 404

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简介： 【Mybatis】（八）高级映射关系（一对一、一对多、多对多）

文章目录

前言

示例

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， MyBatis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"/>
  <!--一对一关联映射-->
  <association property="card" column="card_id" javaType="org.arunner.domain.Card" select="org.arunner.mapper.CardMapper.selectCardById"/>
  </resultMap>
  <!--根据ID查询人员-->
  <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">
  <!--根据ID查询身份证信息-->
  <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"/>
  <!--班级的学生属性，因为一个班级有多个学生，所以该属性是一个集合-->
  <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>
  <!--根据班级ID查询班级-->
  <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"/>
  <!--一对一关系-->
  <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>
  <!--根据班级ID查询班级-->
  <select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
  select * from tb_student where clazz_id = #{id} ;
  </select>
  <!--根据ID查询学生和班级信息-->
  <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">
<!-- namespace指用户自定义的命名空间。 -->
<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"/>
  <!-- 多对一关联映射:association   -->
  <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>
  <!-- 多对多映射的关键:collection   -->
  <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>
  <!-- 注意，如果查询出来的列同名，例如tb_user表的id和tb_order表的id都是id，同名，需要使用别名区分 -->
  <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>
  <!-- 根据userid查询订单 -->
  <select id="selectOrderByUserId" parameterType="int" resultType="org.arunner.domain.Order">
    SELECT * FROM tb_order WHERE user_id = #{id}
  </select>
</mapper>

UserMapper.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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.arunner.mapper.UserMapper">
  <resultMap type="org.arunner.domain.User" id="userResultMap">
  <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"/>
  <!-- 一对多关联映射:collection   -->
  <collection property="orders" javaType="ArrayList"
   column="id" ofType="org.arunner.domain.User"
   select="org.arunner.mapper.OrderMapper.selectOrderByUserId"
   fetchType="lazy">
    <id property="id" column="id"/>
    <result property="code" column="code"/>
    <result property="total" column="total"/>
   </collection>
  </resultMap>
  <select id="selectUserById" parameterType="int" resultMap="userResultMap">
    SELECT * FROM tb_user  WHERE id = #{id}
  </select>
</mapper>

ArticleMapper.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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.arunner.mapper.ArticleMapper">
  <select id="selectArticleByOrderId" parameterType="int" 
  resultType="org.arunner.domain.Article">
    SELECT * FROM tb_article WHERE id IN ( 
  SELECT article_id FROM tb_item WHERE order_id = #{id} 
  ) 
  </select>
</mapper>