Force Hibernate to create foreign key with ON DELE

问题:

When Hibernate (or another JPA implementation) creates foreign key for @OneToMany relationship, is there a way to force it to use ON DELETE CASCADE at the database level? I found that when I use CascadeType.DELETE, Hibernate doesn't do it at the database level, but sends two delete statements (for parent and child records) instead. Or maybe there's a good reason for that?

回答1:

In hibernate, you can use

@OnDelete(action = OnDeleteAction.CASCADE)

on your @OneToMany relationship. This tells hibernate to set ON DELETE CASCADE for the generated foreign key.

Note that this is a hibernate extension and is not specified in the JPA standard.

Use this with caution. When you let database cascade the deletes, these happen outside the control of hibernate so:

• Your second-level cache might become out of sync.
• You cant use on delete listeners on those entities.

I think you should only use this if you have a large collection and performance consideration forces you to let database handles the delete instead of hibernate.

回答2:

They aren't the same thing. JPA Cascade traverses DOWN the object graph whereas database foreign key actions effectively traverse UP the object graph. For example 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