JPA OneToMany not deleting child

Question

Hi,

i have a problem with a simple @OneToMany mapping between a parent and a child entity. All works well, only that child records are not deleted when i remove them from the collection.

The parent:

@Entity
public class Parent {
    @Id
    @Column(name = "ID")
    private Long id;

    @OneToMany(cascade = {CascadeType.ALL}, mappedBy = "parent")
    private Set<Child> childs = new HashSet<Child>();

 ...
}

The child:

@Entity
public class Child {
    @Id
    @Column(name = "ID")
    private Long id;

    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name="PARENTID", nullable = false)
    private Parent parent;

  ...
}

If i now delete and child from the childs Set, it does not get deleted from the database. I tried nullifying the child.parent reference, but that did not work either.

The entities are used in a web application, the delete happens as part of an ajax request. I don't have a list of deleted childs when the save button is pressed, so i can't delete them implicitly.

Any pointers? Thanks in advance for your time

Answer 1

JPA's behaviour is correct (meaning as per the specification): objects aren't deleted simply because you've removed them from a OneToMany collection. There are vendor-specific extensions that do that but native JPA doesn't cater for it.

In part this is because JPA doesn't actually know if it should delete something removed from the collection. In object modeling terms, this is the difference between composition and "aggregation*.

In composition, the child entity has no existence without the parent. A classic example is between House and Room. Delete the House and the Rooms go too.

Aggregation is a looser kind of association and is typified by Course and Student. Delete the Course and the Student still exists (probably in other Courses).

So you need to either use vendor-specific extensions to force this behaviour (if available) or explicitly delete the child AND remove it from the parent's collection.

I'm aware of:

• Hibernate: cascade delete_orphan. See 10.11. Transitive persistence; and
• EclipseLink: calls this "private ownership". See How to Use the @PrivateOwned Annotation.

Answer 2

Here cascade, in the context of remove, means that the children are removed if you remove the parent. Not the association. If you are using Hibernate as your JPA provider, you can do it using hibernate specific cascade.

Answer 3

Thanks to both your replies. I answer myself to post the code that solved my problem. As explained, it is not possible to do what i want with JPA, so i employed the hibernate.cascade annotation, with this, the relevant code in the Parent class now looks like this:

@OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH}, mappedBy = "parent")
@Cascade({org.hibernate.annotations.CascadeType.SAVE_UPDATE,
            org.hibernate.annotations.CascadeType.DELETE,
            org.hibernate.annotations.CascadeType.MERGE,
            org.hibernate.annotations.CascadeType.PERSIST,
            org.hibernate.annotations.CascadeType.DELETE_ORPHAN})
private Set<Child> childs = new HashSet<Child>();

I could not simple use 'ALL' as this would have deleted the parent as well.

Thanks again for your answers.

Answer 4

In addition to cletus' answer, JPA 2.0, final since december 2010, introduces an orphanRemoval attribute on @oneToMany annotations. For more details see this blog entry.

Note that since the spec is relatively new, not all JPA 1 provider have a final JPA 2 implementation. For example, the Hibernate 3.5.0-Beta-2 release does not yet support this attribute.