Jpa配置一对多关系

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

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         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">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.lyf</groupId>
    <artifactId>one-to-more</artifactId>
    <version>1.0-SNAPSHOT</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.1.3.RELEASE</version>
    </parent>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.16.22</version>
        </dependency>
    </dependencies>
</project>

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.*;

/**
 * @Date：2019-04-12
 * @Description：com.lyf.pojo
 * @version：1.0
 */
@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;

    /**
     * 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")
    private Teacher 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<Student> students = new HashSet<>();}

springboot启动类(引导类)

package com.lyf;

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

/**
 * @Date：2019-04-12
 * @Description：com.lyf
 * @version：1.0
 */
@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;

/**
 * @Date：2019-04-12
 * @Description：com.lyf
 * @version：1.0
 */
@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;

/**
 * @Date：2019-04-12
 * @Description：com.lyf
 * @version：1.0
 */
@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

原文地址：https://www.cnblogs.com/a-small-lyf/p/10699326.html