JPA's cascade = REMOVE and Hibernate's @OnDelete used together?

Question

I have inherited a code base on which nearly all relations have the following annotations:

@OneToMany(fetch = FetchType.LAZY, cascade = { CascadeType.REMOVE }, mappedBy = "someThing")
@OnDelete(action = OnDeleteAction.CASCADE)

Now I'm having trouble understanding what @OnDelete does in the first place. Hibernate: OnDelete vs cascade=CascadeType.REMOVE is interesting, but unfortunately doesn't have any answers and the JavaDoc for @OnDelete is particularly worthless.

From the other questions it looks like the OnDelete annotation somehow lets the DB do the cascading, while the cascading directive on @OneToMany let's the ORM do it, but what would ever be the purpose of using them together?

And does @OneToMany's cascade directive really doesn't allow the ORM implementation to generate a DB based cascade anyway?

Answer 1

Let's say you have a one-to-one directional relationship

class House {

    @OneToOne
    Object door;

}

If you use CascadeType.REMOVE then deleting the house will also delete the door.

    @OneToOne(cascade=CascadeType.REMOVE)
    Object door;

If you use @OnDelete then deleting the door will also delete the house.

    @OneToOne
    @OnDelete(action = OnDeleteAction.CASCADE)
    Object door;

Read more here: http://www.ninthavenue.com.au/jpa-cascadetype-remove-vs-hibernate-ondelete