One to many relationship doesn't work Spring boot jpa

Question

I'm trying to create an app using spring boot for practicing my java skills, but I faced with small and stupid problem. I created one to many relationship with UserEntity and LessonEntity. And when I try to add lesson to UserEntity set - nothing is happening. The relation doesn't work.

Here's my code:

UserEntity.java

@Entity
@Table(name = "users", schema = "working")
@Getter @Setter
public class UserEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private long id;
    private String firstName;
    private String lastName;
    private String patronymic;
    @Column(unique = true)
    private String phoneNumber;
    @Column(unique = true)
    private String email;
    private String password;
    private String state;
    @OneToMany(mappedBy = "teacher", cascade = CascadeType.ALL)
    private Set<LessonEntity> lessons;
}

LessonEntity.java

@Entity
@Table(name = "lessons", schema = "working")
@Getter @Setter
public class LessonEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long id;
    @ManyToOne
    @JoinColumn(name = "teacher_id")
    private UserEntity teacher;
}

And here's the code, where I try to add lesson to user:

public void singUpForLesson(UserEntity user, SingUpForLessonModel singUpForLessonModel)
 throws UserNotFoundException {
        UserEntity teacher = userRepo.findById(singUpForLessonModel.getTeacherId()).orElseThrow(UserNotFoundException::new);
        LessonEntity lesson = new LessonEntity();
        teacher.getLessons().add(lesson);
        lessonRepo.save(lesson);
        userRepo.save(teacher);
    }

And here's what I see after saving teacher and lesson:

So what can be the problem? If you know, please tell me, I'd really appreciate it!

check this resource [onetomany-association](https://vladmihalcea.com/the-best-way-to-map-a-onetomany-association-with-jpa-and-hibernate/) — Andrei Lisa, Aug 14 '23 at 11:35
Here's a very nice answer: https://stackoverflow.com/a/28826374/6067302 — TomekK, Aug 14 '23 at 11:39
Related https://stackoverflow.com/questions/13514760/bidirectional-onetoone-relationship-not-symmetric-for-save — Jens Schauder, Aug 14 '23 at 14:21

score 2 · Accepted Answer · answered Aug 14 '23 at 11:42

With a bidirectional relationship you have possibly contradicting information about the state of your relation ship.

This is actually the case in your scenario: The UserEntity has a additional LessonEntity, but the LessonEntity doesn't have a properly set teacher attribute.

You tell JPA with the mappedBy attribute which one to use. This you set to teacher, i.e. LessonEntity.teacher which is still empty.

To fix this set the teacher attribute in your LessonEntity properly.

You can do this manually, in the setter, and I believe Hibernate has a feature to do this automatically for you.

The alternative, which I would prefer is to not have bidirectional relationships at all.

Thank you so much for explanation. Now I understand everything! — Cross, Aug 14 '23 at 12:51

One to many relationship doesn't work Spring boot jpa

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