mybatis一对多关联关系

ゝ一世哀愁。 2022-07-13 02:09 166阅读 0赞

在[《mybatis一对一关联关系》][mybatis]中，我们以班级和教师的例子介绍了一对一的关联关系，现在引入学生类，一个学生对应一个班级，一个班级对应多个学生，来介绍mybatis如何处理一对多的关联关系。  
　　  
　　首先创建表和实体类：

teacher表：

CREATE TABLE teacher( t_id INT PRIMARY KEY AUTO_INCREMENT, t_name VARCHAR(20) );
    INSERT INTO teacher(t_name) VALUES('LS1');
    INSERT INTO teacher(t_name) VALUES('LS2');

![image\_1b4d974o2lm7dfiser8qvj159.png-13.4kB][image_1b4d974o2lm7dfiser8qvj159.png-13.4kB]

class表：

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 class(c_name, teacher_id) VALUES('bj_a', 1);
    INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);

![image\_1b4d4baab1lar1uf61hfj2so1ge613.png-14kB][image_1b4d4baab1lar1uf61hfj2so1ge613.png-14kB]

其中teacher\_id是指向teacher表的外键：

![image\_1b4d98u561fcn30bon314mp1k6vm.png-23.3kB][image_1b4d98u561fcn30bon314mp1k6vm.png-23.3kB]

student表：

CREATE TABLE student( s_id INT PRIMARY KEY AUTO_INCREMENT, s_name VARCHAR(20), class_id INT );
    ALTER TABLE student ADD CONSTRAINT fk_class_id FOREIGN KEY (class_id) REFERENCES class(c_id);   
    INSERT INTO student(s_name, class_id) VALUES('xs_A', 1);
    INSERT INTO student(s_name, class_id) VALUES('xs_B', 1);
    INSERT INTO student(s_name, class_id) VALUES('xs_C', 1);
    INSERT INTO student(s_name, class_id) VALUES('xs_D', 2);
    INSERT INTO student(s_name, class_id) VALUES('xs_E', 2);
    INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);

![image\_1b4d340jksb31n4511659qhbql9.png-12.2kB][image_1b4d340jksb31n4511659qhbql9.png-12.2kB]

其中class\_id是指向class表的外键：

![image\_1b4d3uqo3hmueec1c961m57e7cm.png-17.6kB][image_1b4d3uqo3hmueec1c961m57e7cm.png-17.6kB]

实体类Student，Teacher和Classes：

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

public class Teacher { 
        private int id;
        private String name;
    
        //getters and setters
    
        @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;
        private List<Student> students;
    
        //getters and setters
    
        @Override
        public String toString() {
            return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher + ", students=" + students + "]";
        }
    
    
    
        public Classes(int id, String name, Teacher teacher, List<Student> students) {
            super();
            this.id = id;
            this.name = name;
            this.teacher = teacher;
            this.students = students;
        }
    
        public Classes() {
            super();
        }
    }

编写映射文件classesMapper.xml（关键）

<?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.test6.classesMapper">
    
        
        
        <select id="getClassesById" parameterType="int" resultMap="classesResultMap">
            select * from class c,teacher t,student s where c.teacher_id=t.t_id
            and
            s.class_id=c.c_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>
            
            <collection property="students" ofType="Student">
                <id property="id" column="s_id" />
                <result property="name" column="s_name" />
            </collection>
        </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>
            <collection property="students" column="c_id" select="getStudentByClassId"></collection>
        </resultMap>
        <select id="getTeacherById" parameterType="int" resultType="Teacher">
            select t_name name from teacher where t_id=#{id}
        </select>
        <select id="getStudentByClassId" parameterType="int" resultType="Student">
            select s_id id,s_name name from student where class_id=#{id}
        </select>
    </mapper>

这里提供了两种方法处理一对多的关联关系，和一对一中类似，分别是联表查询和分两次查询。关键点在于resultMap节点中的collection节点，注意在联表查询时，需要通过collection节点的ofType属性指定返回的集合的元素类型。

[mybatis]: http://blog.csdn.net/xiangwanpeng/article/details/53760251
[image_1b4d974o2lm7dfiser8qvj159.png-13.4kB]: /images/20220713/d9d5a695067444d7a33cf9c1c378e9d9.png
[image_1b4d4baab1lar1uf61hfj2so1ge613.png-14kB]: /images/20220713/05603addc33a46e89db8cfed5c115234.png
[image_1b4d98u561fcn30bon314mp1k6vm.png-23.3kB]: /images/20220713/d3178f5f53754bf7acb9c3a91d949dff.png
[image_1b4d340jksb31n4511659qhbql9.png-12.2kB]: /images/20220713/fec62fdd56554d9684f09fc0f88a34ff.png
[image_1b4d3uqo3hmueec1c961m57e7cm.png-17.6kB]: /images/20220713/da92a3ee779e4675b34ea62d06ff32f2.png