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

 CREATE TABLE [dbo].[t_teacher](
     [id] [int] IDENTITY(1,1) NOT NULL,
     [name] [nvarchar](30) NULL,
     [title] [nvarchar](30) NULL,
 PRIMARY KEY CLUSTERED
 (
     [id] ASC
 )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
 ) ON [PRIMARY]

 GO

 CREATE TABLE [dbo].[t_student](
     [id] [int] IDENTITY(1,1) NOT NULL,
     [name] [nvarchar](30) NULL,
     [major] [nvarchar](30) NULL,
     [supervisor_id] [int] NULL,
 PRIMARY KEY CLUSTERED
 (
     [id] ASC
 )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
 ) 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

 <?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="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" javaType="Teacher">
             <id property="id" column="t_id" />
             <result property="name" column="t_name"/>
             <result property="title" column="title"/>
         </association>
     </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>

teacher model

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

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

@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