MyBatis關聯對映關係，一對一，一對多，多對多
阿新 • • 發佈：2018-11-11
“縱許悠揚度朱戶，終愁人影隔窗紗”
前言
客觀世界中的物件很少有孤立存在的，例如班級，往往與班級的學生存在關聯關係，如果得到某個班級的例項，那麼應該可以直接獲取班級對應的全部學生。反過來，如果已經得到個學生的例項，那麼也應該可以訪問該學生對應的班級。這種例項之間的互相訪問就是關聯關係。
關聯關係是面向物件分析、面向物件設計最重要的知識， My Batis完全可以理解這種關聯關係，如果對映得當， My Batis的關聯對映將可以大大簡化持久層資料的訪問。
示例
這裡只給出對映檔案（XML檔案）中的關鍵程式碼
- 一對一關係
例如：一個人對應一個身份證號
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"/> <!--一對一關聯對映--> <association property="card" column="card_id" javaType="org.arunner.domain.Card" select="org.arunner.mapper.CardMapper.selectCardById"/> </resultMap> <!--根據ID查詢人員--> <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"> <!--根據ID查詢身份證資訊--> <select id="selectCardById" parameterType="int" resultType="org.arunner.domain.Card"> select * from tb_card where id = #{id} ; </select> </mapper>
- 一對多關係
例如：一個班級對應多個學生
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"/>
<!--班級的學生屬性，因為一個班級有多個學生，所以該屬性是一個集合-->
<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>
<!--根據班級ID查詢班級-->
<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"/>
<!--一對一關係-->
<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>
<!--根據班級ID查詢班級-->
<select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
select * from tb_student where clazz_id = #{id} ;
</select>
<!--根據ID查詢學生和班級資訊-->
<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>
- 多對多關係
例如：訂單、商品和使用者的關係
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指使用者自定義的名稱空間。 -->
<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"/>
<!-- 多對一關聯對映: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>
<!-- 多對多對映的關鍵: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，如果查詢出來的列同名，例如tb_user表的id和tb_order表的id都是id，同名，需要使用別名區分 -->
<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>
<!-- 根據userid查詢訂單 -->
<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指使用者自定義的名稱空間。 -->
<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"/>
<!-- 一對多關聯對映: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指使用者自定義的名稱空間。 -->
<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>