双向 OneToMany 与 OneToOne 关联与 mappedBy 属性

Question

I defined entities with @OneToOne bidirectional relationship.

Owner site of relationship ( Child class):

@OneToOne
@JoinColumn(name = "owner_id")
private Parent parent;

Other site ( Parent class):

@OneToOne(mappedBy = "parent")
private Child child;

When I try to persist Parent who has child set (which is not persisted) then I got exception

• TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing

which looks understandable.

But when I defined entities with bidirectional OneToMany/ManyToOne association like:

The owning site of this relationship:

@ManyToOne
@JoinColumn(name = "owner_id")
private Parent parent;

The other side of the relationship:

@OneToMany(mappedBy = "parent")
private Set<Child> childrens = new HashSet<>();

and try to persist Parent who has children collection set (which are not persisted) then I can do it and Hibernate persist only Parent.

For me it looks strange because this behavior is not deterministic - I know that this example is not correctly and I can use CascadeType.ALL or persist it correctly. Do you know why it looks that? Is that Hibernate use different algorithms to persist entities in @OneToOne/@OneToMany relationship

Answer 1

If you use cascade = PERSIST on the @OneToMany you should see the same error. The difference is just that Hibernate does not try to "manage" *-to-many associations on the inverse side ie where the mappedBy is present.