mybatis一对一

比眉伴天荒 2022-06-08 13:57 206阅读 0赞

## 前言 ##

如果在一个实体bean里包含其它实体bean，如一个班主任只属于一个班级，一个班级也只能有一个班主任，那么在Teacher实体bean中包含Class实体bean，这时mybatis在查询到数据后会直接映射到Teacher实体中并完成对Class的映射。下面具体讲一下映射文件怎么写。

## 具体一对一 ##

1.创建班主任teacher表

CREATE TABLE `teacher` ( `teacher_no` int(11) NOT NULL, `teacher_name` varchar(255) NOT NULL, `class_no` int(11) NOT NULL, PRIMARY KEY (`teacher_no`) )

创建班级class表

CREATE TABLE `class` ( `class_no` int(11) NOT NULL, `class_name` varchar(255) NOT NULL, PRIMARY KEY (`class_no`) )

表中数据就自己添加吧

2.创建Classes实体类

public class Classes {
    
        private int classNo;//课程号
    
        private String className;//课程名
    
        public int getClassNo() {
            return classNo;
        }
    
        public void setClassNo(int classNo) {
            this.classNo = classNo;
        }
    
        public String getClassName() {
            return className;
        }
    
        public void setClassName(String className) {
            this.className = className;
        }
    
    }

创建Teacher实体类

public class Teacher {
    
        private int teacherNo;//教师号
    
        private String teacherName;//教师名
    
        private Classes classes;
    
        public int getTeacherNo() {
            return teacherNo;
        }
    
        public void setTeacherNo(int teacherNo) {
            this.teacherNo = teacherNo;
        }
    
        public String getTeacherName() {
            return teacherName;
        }
    
        public void setTeacherName(String teacherName) {
            this.teacherName = teacherName;
        }
    
        public Classes getClasses() {
            return classes;
        }
    
        public void setClasses(Classes classes) {
            this.classes = classes;
        }
    
    }

可以看到Teacher类中包含Classes实体类。

3.mybatis映射文件配置

<?xml version="1.0" encoding="GBK"?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    
    <mapper namespace="com.mycompany.dao.HelloWorldDao">
    
        
        <resultMap type="com.mycompany.entity.Teacher" id="getClassMap">
            <id property="teacherNo" column="teacher_no"/>   
            <result property="teacherName" column="teacher_name"/>
            <association property="classes" javaType="com.mycompany.entity.Classes">   
                <id property="classNo" column="class_no"/>  
                <result property="className" column="class_name"/>
            </association>
        </resultMap>
    
        <select id="getTeacherDetail" parameterType="int" resultMap="getClassMap">
            select * from class c, teacher t  where c.teacher_no = t.teacher_no and c.teacher_no=#{id}
        </select>
    
    </mapper>

4.java代码

List<Teacher> list = helloWorldDao.getTeacherDetail(id);

所以，不论where条件是什么，查询出多少条Teacher，每条Teacher都对应一条Classes。

## 后记 ##

一对一的关键就在于实体类中要包含另一个实体类，而且mapper映射文件中要用到association子标签，该标签用于一对一的关联查询

*  property 对象属性的名称
 *  javaType 对象属性的类型
 *  column 所对应的外键字段名称
 *  select 使用另一个查询封装的结果