mybatis 中一对多关系简单应用

1：应用场景

　　主要需要实现是一个实体类中存在一个字段，该字段为一个集合，集合存放的是另外一个实体类类型

　　　　如教室类（Class）中有一个学生类（student）的集合，即表示一个教室中 存在多个学生

　　项目保存路径： D:\海同\mybatis\9.16\FirstMybatis

2：相关代码

　　创建数据库中表语句　

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);

CREATE TABLE student(
    s_id INT PRIMARY KEY AUTO_INCREMENT, 
    s_name VARCHAR(20), 
    class_id INT
);
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);

　　2：实体类

package com.yuwenhui.entity;

/**
 * Created by Administrator on 2017/9/18 0018.
 */
public class Student {
    private Integer id;
    private String name;

    public Student() {
    }

    public Student(Integer id, String name) {
        this.id = id;
        this.name = name;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

package com.yuwenhui.entity;

import java.util.List;

/**
 * Created by Administrator on 2017/9/18 0018.
 */
public class ClassOne2Many {
    private Integer id;
    private String name;
    private Teacher teacher;
    private List<Student> students;

    public ClassOne2Many() {
    }

    public ClassOne2Many(Integer id, String name, Teacher teacher, List<Student> students) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
        this.students = students;
    }

    @Override
    public String toString() {
        return "ClassOne2Many{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", teacher=" + teacher +
                ", students=" + students +
                '}';
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }
}

　　映射文件

<?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.yuwenhui.entity.classOne2ManyMapper">
    <!--方法一：多表联合查询-->
    <select id="sleectClassOne2Many1" resultMap="ClassOne2ManyResultMapper1" parameterType="Integer">
        SELECT * FROM class c,teacher t,student s WHERE c.c_id = s.class_id AND c.teacher_id = t.t_id AND c.c_id = #{id};
    </select>
    <resultMap id="ClassOne2ManyResultMapper1" type="_ClassOne2Many">
        <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>
        <!--表示一对多，collection 表示一个集合-->
        <collection property="students" ofType="_Student" >
            <id property="id" column="s_id"/>
            <result property="name" column="s_name"/>
        </collection>
    </resultMap>

    <!--方法二： 执行多次sql语句-->
    <select id="sleectClassOne2Many2" resultMap="ClassOne2ManyResultMapper2" parameterType="Integer">
        SELECT * FROM class c WHERE  c.c_id = #{id};
    </select>
    <resultMap id="ClassOne2ManyResultMapper2" type="_ClassOne2Many">
        <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>
        <!--表示一对多-->
        <collection property="students" ofType="_Student" column="c_id" select="selectStudent2">
            <!--<id property="id" column="s_id"/>-->
            <!--<result property="name" column="s_name"/>-->
        </collection>
    </resultMap>
    <select id="selectStudent2" resultMap="studentResultMapper" parameterType="Integer">
        SELECT * FROM student WHERE class_id = #{id};
    </select>
    <resultMap id="studentResultMapper" type="_Student">
        <id property="id" column="s_id"/>
        <result property="name" column="s_name"/>
    </resultMap>
</mapper>

　　测试：

@Before
    public void befor() {
        String resource = "conf.xml";
//        InputStream resourceAsStream = TestMybatis.class.getClassLoader().getResourceAsStream(resource);
        Reader reader = null;
        try {

            reader = Resources.getResourceAsReader(resource);
        } catch (IOException e) {
            e.printStackTrace();
        }
        sessionFactory = new SqlSessionFactoryBuilder().build(reader);
        session = sessionFactory.openSession();
    }

    /**
     * 测试一对多关系
     * 一个教室有多个学生
     * 多表联合查询
     */
    @Test
    public void testOne2Many(){
        String statement = "com.yuwenhui.entity.classOne2ManyMapper"+".sleectClassOne2Many1";
        ClassOne2Many clazz = session.selectOne(statement,1);
        System.out.println(clazz);
    }

    /**
     * 测试一对多关系
     * 一个教室有多个学生
     * 多次发送sql语句
     */
    @Test
    public void testOne2Many2(){
        String statement = "com.yuwenhui.entity.classOne2ManyMapper"+".sleectClassOne2Many2";
        ClassOne2Many clazz = session.selectOne(statement,2);
        System.out.println(clazz);
    }

 @After
    public void after() {
        session.close();
    }