MyBatis：一对一关联查询

MyBatis从入门到放弃三：一对一关联查询

前言

简单来说在mybatis.xml中实现关联查询实在是有些麻烦，正是因为起框架本质是实现orm的半自动化。 那么mybatis实现一对一的关联查询则是使用association属性和resultMap属性。

搭建开发环境

创建student表、teacher表来搭建一对一的关联查询场景，student表添加外键supervisor_id实现和teacher表的关联

 1 CREATE TABLE [dbo].[t_teacher](
 2     [id] [int] IDENTITY(1,1) NOT NULL,
 3     [name] [nvarchar](30) NULL,
 4     [title] [nvarchar](30) NULL,
 5 PRIMARY KEY CLUSTERED
 6 (
 7     [id] ASC
 8 )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
 9 ) ON [PRIMARY]
10
11 GO

 1 CREATE TABLE [dbo].[t_student](
 2     [id] [int] IDENTITY(1,1) NOT NULL,
 3     [name] [nvarchar](30) NULL,
 4     [major] [nvarchar](30) NULL,
 5     [supervisor_id] [int] NULL,
 6 PRIMARY KEY CLUSTERED
 7 (
 8     [id] ASC
 9 )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
10 ) ON [PRIMARY]

一对一关联查询

一对一关联的关键是在mapper.xml中创建resultMap。  如下代码看到了在studentResultMap中添加了属性association，property是在model类中的外键属性名称，别忘记指定JavaType

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <!DOCTYPE mapper
 3         PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
 4         "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 5 <mapper namespace="com.autohome.mapper.Student">
 6     <resultMap id="studentResultMap" type="Student">
 7         <id property="id" column="id"/>
 8         <result property="name" column="name"/>
 9         <result property="major" column="major"/>
10         <association property="supervisor" javaType="Teacher">
11             <id property="id" column="t_id" />
12             <result property="name" column="t_name"/>
13             <result property="title" column="title"/>
14         </association>
15     </resultMap>
16
17
18     <select id="getStudentById" parameterType="int" resultMap="studentResultMap">
19         SELECT st.id,st.name,st.major,
20                t.id t_id,t.name t_name,t.title
21         FROM t_student st inner join t_teacher t on st.supervisor_id=t.id
22         where st.id=#{id}
23     </select>
24 </mapper>

teacher model

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public class Teacher {     private int id;     private String name;     private String title;       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 String getTitle() {         return title;     }       public void setTitle(String title) {         this.title = title;     } }

student model

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public class Student {     private int id;     private String name;     private String major;       private Teacher supervisor;       public Teacher getSupervisor() {         return supervisor;     }       public void setSupervisor(Teacher supervisor) {         this.supervisor = supervisor;     }         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 String getMajor() {         return major;     }       public void setMajor(String major) {         this.major = major;     } }

　　

单元测试

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@Test     public void getStudentById(){         SqlSession sqlSession=null;         try{             sqlSession=sqlSessionFactory.openSession();               Student student = sqlSession.selectOne("com.autohome.mapper.Student.getStudentById",1);             System.out.println("id:"+student.getId()+",name:"+student.getName()+",导师姓名:"+student.getSupervisor().getName()+",导师职称:"+student.getSupervisor().getTitle());         }catch(Exception e){             e.printStackTrace();         }finally {             sqlSession.close();         }     }

附实现截图

嵌套resultMap

如上的resultmap用起来总是不方便的，万一后续再到其他关联查询需要用到teacher表呢，那么我们把teacherResultMap单独拿出来，不仅是resultMap可以嵌套，sql语句也可以嵌套。

分别创建studentResultMap和suprvisorResultMap。后在studentResultMap的association中使用resultMap引用supervisorResultMap。

<mapper namespace="com.autohome.mapper.Student">

    <resultMap id="studentResultMap" type="Student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="major" column="major"/>
        <association property="supervisor" resultMap="suprvisorResultMap"/>
    </resultMap>

    <resultMap id="suprvisorResultMap" type="Teacher">
        <id property="id" column="t_id" />
        <result property="name" column="t_name"/>
        <result property="title" column="title"/>
    </resultMap>

    <select id="getStudentById" parameterType="int" resultMap="studentResultMap">
        SELECT st.id,st.name,st.major,
               t.id t_id,t.name t_name,t.title
        FROM t_student st inner join t_teacher t on st.supervisor_id=t.id
        where st.id=#{id}
    </select>
</mapper>

实现结果和上面相同。