Hibernate deletes one child enitity too much, after exiting Transactional context-CodePudding

I've encountered a strange problem in a simple Spring Boot application, where I update a parent entity, delete all of its children and then create new ones.

After saving, the resulting entity looks fine and has all of the new children, but when I query it again, I find that one of the child entities is lost!

The quick and dirty solution for this would be to split the code into two transactions, but I want to understand the cause the orphan removal to act like this.

Here's the service code:

@Service
public class ParentService {
    private final EntityManager em;

    public ParentEntity getParent(UUID parentId) {
        return em.createQuery(
                        "SELECT p "  
                                "from ParentEntity p "  
                                "JOIN FETCH p.children "  
                                "WHERE p.id = :parentId", ParentEntity.class)
                .setParameter("parentId", parentId)
                .getSingleResult();
    }

    @Transactional
    public ParentEntity resetChildren(UUID parentId) {
        var parent = getParent(parentId);

        parent.getChildren().clear();
        addChildren(parent, 2);

        em.persist(parent);
        return parent;
    }

    private void addChildren(ParentEntity parent, int childCount) {
        for (var i = 0; i < childCount; i  ) {
            parent.addChildren(new ChildEntity());
        }
    }
}

The SQL output after resetting children and the fetching the parent again is this:

select parententi0_.id as id1_1_0_, children1_.parent_id as parent_i2_0_1_, children1_.id as id1_0_1_, children1_.id as id1_0_2_, children1_.parent_id as parent_i2_0_2_ from parent parententi0_ left outer join child children1_ on parententi0_.id=children1_.parent_id where parententi0_.id=?
insert into child (parent_id, id) values (?, ?)
insert into child (parent_id, id) values (?, ?)
delete from child where id=?
delete from child where id=?
delete from child where id=? <-- One extra delete

select parententi0_.id as id1_1_0_, children1_.parent_id as parent_i2_0_1_, children1_.id as id1_0_1_, children1_.id as id1_0_2_, children1_.parent_id as parent_i2_0_2_ from parent parententi0_ left outer join child children1_ on parententi0_.id=children1_.parent_id where parententi0_.id=?

The entities look like this:

The parent

@Entity
@Table(name = "parent")
public class ParentEntity {
    @Id
    @GeneratedValue
    private UUID id;

    @OneToMany(mappedBy = "parent", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<ChildEntity> children = new HashSet<>();
    ...

    public void addChildren(ChildEntity child) {
        this.children.add(child);
        child.setParent(this);
    }
    ...
}

And the child

@Entity
@Table(name = "child")
public class ChildEntity {

    @Id
    @GeneratedValue
    private UUID id;

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "parent_id")
    private ParentEntity parent;
    ...
}

An important note to this, that in my use case, the id of these entities is a UUID. I can't get it working with any numeric ids

The code repository with a unit test can be found here

An interesting thing happens, if I decide to add 1 child, instead of two or more, the parent itself is deleted! Because of this, it feels like I'm looking at a bug in Hibernate.

CodePudding user response：

The problem was that I had cascade on the child side, which lead to very very weird results. I don't want parent to be deleted on cascade, so I don't need that.

The fix was to change the child to this:

@Entity
@Table(name = "child")
public class ChildEntity {

    @Id
    @GeneratedValue
    private UUID id;

    @ManyToOne // <---- No more cascade!
    @JoinColumn(name = "parent_id")
    private ParentEntity parent;
    ...
}