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

       简单来讲在mybatis.xml中实现关联查询实在是有些麻烦，正是由于起框架本质是实现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

      建立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中建立resultMap。  以下代码看到了在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用起来老是不方便的，万一后续再到其余关联查询须要用到teacher表呢，那么咱们把teacherResultMap单独拿出来，不只是resultMap能够嵌套，sql语句也能够嵌套。

   分别建立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。