（转载）Jpa配置一对多关系

链接： https://www.cnblogs.com/a-small-lyf/p/10699326.html

 

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

         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    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=08186912
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true
spring.jpa.database=mysql

实体类

package com.lyf.pojo;

import lombok.Data;

import javax.persistence.*;


@Data
@Entity
@Table(name = "tb_student")
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "s_id")
    private Long sId;
    @Column(name = "s_name")
    private String sName;

    
    @ManyToOne(targetEntity = Teacher.class)
    @JoinColumn(name = "s_t_id",referencedColumnName = "t_id")
    private Teacher teacher;
}

package com.lyf.pojo;

import lombok.Data;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;


@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
    
    @OneToMany(targetEntity = Student.class)
    @JoinColumn(name = "s_t_id",referencedColumnName = "t_id")
    private Set students = new HashSet<>();
}

springboot启动类(引导类)

package com.lyf;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;


@SpringBootApplication
public class App {
    public static void main(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

 

package com.lyf;

import com.lyf.dao.StudentDao;
import com.lyf.dao.TeacherDao;
import com.lyf.pojo.Student;
import com.lyf.pojo.Teacher;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;


@SpringBootTest
@RunWith(SpringRunner.class)
public class OneToMoreTest {

    @Autowired
    private TeacherDao teacherDao;
    @Autowired
    private StudentDao studentDao;
    @Test
    public void addTest(){
        Student student = new Student();
        student.setSName("老篮孩i");
        Teacher teacher = new Teacher();
        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

 

package com.lyf;

import com.lyf.dao.StudentDao;
import com.lyf.dao.TeacherDao;
import com.lyf.pojo.Student;
import com.lyf.pojo.Teacher;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;


@SpringBootTest
@RunWith(SpringRunner.class)
public class OneToMoreTest {

    @Autowired
    private TeacherDao teacherDao;
    @Autowired
    private StudentDao studentDao;
    @Test
    public void addTest(){
        Student student = new Student();
        student.setSName("老篮孩i");
        Teacher teacher = new Teacher();
        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

@Test
    public void addTest1(){
        Student student = new Student();
        student.setSName("老篮孩i1");
        Teacher teacher = new Teacher();
        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