Parent-child relationships are a special case of more general 1:M associations. They are simply part-whole relationships and for Hibernate it is not meaningful that parts (children) should exist without belonging to a whole (parent).
Parent-child association is specified with the orphanRemoval attribute of the @OneToMany annotation. Hibernate
achieves this by employing a special persistent collection implementation.
The first thing you should be aware of is that when a child entity is just removed from its containing collection (e.g.,
children.remove(child)), Hibernate issues an SQL DELETE statement at flush time for that child entity without expecting
an explicit session.delete(child) call. Remove operations are tracked by that persistent collection and removal of a
child means that it becomes orphaned, and orphaned children should be immediately deleted.
Another important point is that when you want to get rid of all of the children in a collection, just call
children.clear(). If you try to set the parent’s collection property value to NULL (for example,
parent.setChildren(null)), you will get an HibernateException with the following message:
“A collection with cascade="all-delete-orphan" was no longer referenced by the owning entity instance.”
This message tells us that the collection instance used to manage the parent-child relationship is de-referenced by its
parent. In other words, the connection between the two is just lost. However, this exception only arises when you perform
the NULL set operation on the attached parent entity. If the parent is detached, you can safely set its children
property to NULL and then reattach it to the session. Reattachment will cause all of its children to be deleted as well.