java 多对多关系好的解决方案 java一对多关系

在网上查了很多关于jpa的一对多表关联的操作,踩了很多坑,今天终于解决了

下面上一下我自己的代码,记录一下

老师和学生(一对多关系)

首先搭建环境,添加依赖包

4.0.0
com.lyf
one-to-more
1.0-SNAPSHOT
org.springframework.boot
spring-boot-starter-parent
2.1.3.RELEASE
org.springframework.boot
spring-boot-starter-data-jpa
mysql
mysql-connector-java
5.1.47
org.springframework.boot
spring-boot-starter-test
org.projectlombok
lombok
1.16.22

编写数据库配置文件

spring.datasource.url=jdbc:mysql://localhost:3306/jpa?serverTimezone=UTC
spring.datasource.username=root
spring.datasource.password=08186912spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=truespring.jpa.database=mysql

实体类

packagecom.lyf.pojo;importlombok.Data;import javax.persistence.*;/*** @Date：2019-04-12
* @Description：com.lyf.pojo
*@version：1.0*/@Data
@Entity
@Table(name= "tb_student")public classStudent {
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
@Column(name= "s_id")privateLong sId;
@Column(name= "s_name")privateString sName;/*** 多个学生对应一个老师
* 注解形式配置多对一
* 1,配置表关系
* 2,配置外键*/@ManyToOne(targetEntity= Teacher.class)
@JoinColumn(name= "s_t_id",referencedColumnName = "t_id")privateTeacher teacher;
}
package com.lyf.pojo;
import lombok.Data;
import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;
/**
* @Date：2019-04-12
* @Description：教师和学生是一对多
*/
@Data
@Entity
@Table(name = "tb_teacher")
public class Teacher {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "t_id")
private Long tId;
@Column(name = "t_name")
private String tName;
//配置老师和学生一对多
/**
* 注解配置多表关系
* 1,声名关系
* 2,配置外键,或者中间表
* OneToMany配置一对多
* targetEntity设置对应的实体类的类型
* JoinColumn 配置外键
* name:外键的名称,
* referencedColumnName参照的主表的主键字段名称
*/
@OneToMany(targetEntity = Student.class)
@JoinColumn(name = "s_t_id",referencedColumnName = "t_id")
private Set students = new HashSet<>();
}
springboot启动类(引导类)
packagecom.lyf;importorg.springframework.boot.SpringApplication;importorg.springframework.boot.autoconfigure.SpringBootApplication;/*** @Date：2019-04-12
* @Description：com.lyf
*@version：1.0*/@SpringBootApplicationpublic classApp {public static voidmain(String[] args) {
SpringApplication.run(App.class,args);
}
}

启动引导类,查看数据库会发现表生成成功

dao层代码就不上了,继承JpaRepository就行了

接下来我们进行保存操作

packagecom.lyf;importcom.lyf.dao.StudentDao;importcom.lyf.dao.TeacherDao;importcom.lyf.pojo.Student;importcom.lyf.pojo.Teacher;importorg.junit.Test;importorg.junit.runner.RunWith;importorg.springframework.beans.factory.annotation.Autowired;importorg.springframework.boot.test.context.SpringBootTest;importorg.springframework.test.context.junit4.SpringRunner;/*** @Date：2019-04-12
* @Description：com.lyf
*@version：1.0*/@SpringBootTest
@RunWith(SpringRunner.class)public classOneToMoreTest {
@AutowiredprivateTeacherDao teacherDao;
@AutowiredprivateStudentDao studentDao;
@Testpublic voidaddTest(){
Student student= newStudent();
student.setSName("老篮孩i");
Teacher teacher= newTeacher();
teacher.setTName("刘老师");//关联学生和老师,添加学生信息时，还需添加外键的值
student.setTeacher(teacher);
studentDao.save(student);
teacherDao.save(teacher);
}
}

结果报错了,发现我是先保存的学生信息，再保存的老师信息,此时数据库中并没有老师的信息,给学生关联老师信息肯定是有问题的

报错信息

org.springframework.dao.InvalidDataAccessApiUsageException:
org.hibernate.TransientPropertyValueException:
object references an unsaved transient instance - save the transient instance before flushing : com.lyf.pojo.Student.teacher -> com.lyf.pojo.Teacher;
nested exception is java.lang.IllegalStateException: org.hibernate.TransientPropertyValueException:
object references an unsaved transient instance - save the transient instance before flushing :
com.lyf.pojo.Student.teacher -> com.lyf.pojo.Teacher

学生表记录插入了,老师表是空的

改成

packagecom.lyf;importcom.lyf.dao.StudentDao;importcom.lyf.dao.TeacherDao;importcom.lyf.pojo.Student;importcom.lyf.pojo.Teacher;importorg.junit.Test;importorg.junit.runner.RunWith;importorg.springframework.beans.factory.annotation.Autowired;importorg.springframework.boot.test.context.SpringBootTest;importorg.springframework.test.context.junit4.SpringRunner;/*** @Date：2019-04-12
* @Description：com.lyf
*@version：1.0*/@SpringBootTest
@RunWith(SpringRunner.class)public classOneToMoreTest {
@AutowiredprivateTeacherDao teacherDao;
@AutowiredprivateStudentDao studentDao;
@Testpublic voidaddTest(){
Student student= newStudent();
student.setSName("老篮孩i");
Teacher teacher= newTeacher();
teacher.setTName("刘老师");//关联学生和老师,添加学生信息时，还需添加外键的值
student.setTeacher(teacher);//要先保存主表信息
teacherDao.save(teacher);
studentDao.save(student);
}
}

控制台信息,很显然成功了

Hibernate: alter table tb_student add constraint FKsojy7bicq68v21slcq9mtwtou foreign key (s_t_id) references tb_teacher (t_id)2019-04-12 23:29:42.036 INFO 10980 --- [ main] j.LocalContainerEntityManagerFactoryBean : Initialized JPA EntityManagerFactory for persistence unit ‘default‘
2019-04-12 23:29:42.748 INFO 10980 --- [ main] com.lyf.OneToMoreTest : Started OneToMoreTest in 7.77 seconds (JVM running for 9.806)
Hibernate: insert into tb_teacher (t_name) values (?)
Hibernate: insert into tb_student (s_name, s_t_id) values (?, ?)

查看数据库也没有问题

同样我们通过Teacher表也能完成关联操作,保存也是没有问题的

@Testpublic voidaddTest1(){
Student student= newStudent();
student.setSName("老篮孩i1");
Teacher teacher= newTeacher();
teacher.setTName("刘老师1");//通过主表来添加关联
teacher.getStudents().add(student);
studentDao.save(student);
teacherDao.save(teacher);
}

控制打印信息

Hibernate: insert into tb_student (s_name, s_t_id) values (?, ?)
Hibernate: insert into tb_teacher (t_name) values (?)
Hibernate: update tb_student set s_t_id=? where s_id=?

学生类和老师类都添加类外键的配置,都具备外键的维护,那么我们这里可以通过学生找到老师，也能通过老师找到学生,这是一种双向关系

如果只配置一方,那就是单向的关系，只能通过指定的一方找到另一方