One of the very first things that new users try to do with Hibernate is to model a parent / child type
relationship. There are two different approaches to this. For various reasons the most convenient
approach, especially for new users, is to model both Parent and Child
as entity classes with a <one-to-many> association from Parent
to Child. (The alternative approach is to declare the Child as a
<composite-element>.) Now, it turns out that default semantics of a one to many
association (in Hibernate) are much less close to the usual semantics of a parent / child relationship than
those of a composite element mapping. We will explain how to use a bidirectional one to many
association with cascades to model a parent / child relationship efficiently and elegantly.
It's not at all difficult!
Hibernate collections are considered to be a logical part of their owning entity; never of the contained entities. This is a crucial distinction! It has the following consequences:
When we remove / add an object from / to a collection, the version number of the collection owner is incremented.
If an object that was removed from a collection is an instance of a value type (eg, a composite element), that object will cease to be persistent and its state will be completely removed from the database. Likewise, adding a value type instance to the collection will cause its state to be immediately persistent.
On the other hand, if an entity is removed from a collection (a one-to-many or many-to-many association), it will not be deleted, by default. This behaviour is completely consistent - a change to the internal state of another entity should not cause the associated entity to vanish! Likewise, adding an entity to a collection does not cause that entity to become persistent, by default.
Instead, the default behaviour is that adding an entity to a collection merely creates a link between the two entities, while removing it removes the link. This is very appropriate for all sorts of cases. Where it is not appropriate at all is the case of a parent / child relationship, where the life of the child is bound to the life cycle of the parent.