mybatis一对一关联关系

野性酷女 2022-07-13 01:59 236阅读 0赞

这篇文章介绍mybatis如何处理一对一的关联关系，假设现在有班级类（表）和教师类（表），一个教师对应一个班级，一个班级也只对应一个教师。  
　　  
首先创建表并插入数据：

CREATE TABLE teacher( t_id INT PRIMARY KEY AUTO_INCREMENT, t_name VARCHAR(20) );
    CREATE TABLE class( c_id INT PRIMARY KEY AUTO_INCREMENT, c_name VARCHAR(20), teacher_id INT );
    ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);   
    
    INSERT INTO teacher(t_name) VALUES('LS1');
    INSERT INTO teacher(t_name) VALUES('LS2');
    
    INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1);
    INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);

![image\_1b4d0n484ohg1bg8m701pkegv59.png-15.1kB][image_1b4d0n484ohg1bg8m701pkegv59.png-15.1kB]

![image\_1b4d0ndqt1d001otb3bd1kbo6tfm.png-15.2kB][image_1b4d0ndqt1d001otb3bd1kbo6tfm.png-15.2kB]

其中，class表中的teacher\_id是指向teacher表的外键：  
![image\_1b4d0sebt4d3e9r1fhjgm7e813.png-17.9kB][image_1b4d0sebt4d3e9r1fhjgm7e813.png-17.9kB]

创建实体类：

public class Teacher { 
        private int id;
        private String name;
    
        @Override
        public String toString() {
            return "Teacher [id=" + id + ", name=" + name + "]";
        }
    
        public Teacher(int id, String name) {
            super();
            this.id = id;
            this.name = name;
        }
    
        public Teacher() {
            super();
        }
    }

public class Classes { 
        private int id;
        private String name;
        private Teacher teacher;
    
        @Override
        public String toString() {
            return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher + "]";
        }
    
        public Classes(int id, String name, Teacher teacher) {
            super();
            this.id = id;
            this.name = name;
            this.teacher = teacher;
        }
    
        public Classes() {
            super();
        }
    }

主要的处理过程在Classes类的映射文件classesMapper.xml中：

<?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.mybatis.test5.classesMapper">
        
    
        
        <select id="getClassesById" parameterType="int" resultMap="classesResultMap">
            select * from class c,teacher t where c.teacher_id=t.t_id and
            c.c_id=#{id}
        </select>
        <resultMap type="Classes" id="classesResultMap">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="Teacher">
        <id property="id" column="t_id"/>
        <result property="name" column="t_name"/>
        </association>
        </resultMap>
    
        
        
    
        <select id="getAllClasses" resultMap="allClassesResultMap">
        select * from class
        </select>
        <resultMap type="Classes" id="allClassesResultMap">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" select="getTeacherById">
        </association>
        </resultMap>
        <select id="getTeacherById" parameterType="int" resultType="Teacher">
        select t_name name from teacher where t_id=#{id}
        </select>
    </mapper>

这里介绍了两种方法，第一种是通过联表查询,获取每条class记录执行一次select语句：

SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=?;

另一种方法是通过两次查询，先查找class，再根据查找到的class的teacher\_id查询teacher，即获取每条class记录执行两次select语句：

SELECT * FROM class WHERE c_id=?;
    SELECT * FROM teacher WHERE t_id=?;

其中的一个重点是resultMap节点中的association节点，它将查询到的列值赋给Classes类的Teacher属性类的对应字段。另外，在第二种方法中，association节点的select属性指定了第二次查询的select节点。  
association节点的属性如下：

*  property：对象属性的名称
 *  javaType：对象属性的类型
 *  column：对应的外键字段名称
 *  select：使用的另一个查询的id

[image_1b4d0n484ohg1bg8m701pkegv59.png-15.1kB]: /images/20220713/df4eda74d64a4e0b80e5a09562aa68ed.png
[image_1b4d0ndqt1d001otb3bd1kbo6tfm.png-15.2kB]: /images/20220713/54b3b47e2fe84350b07efc78d8e8c9eb.png
[image_1b4d0sebt4d3e9r1fhjgm7e813.png-17.9kB]: /images/20220713/8402b6075f0c43df90da20d1c7999bf7.png