3.10. Transitive persistence

It is quite cumbersome to save, delete, or reattach individual objects, especially if you deal with a graph of associated objects. A common case is a parent/child relationship. Consider the following example:

If the children in a parent/child relationship would be value typed (e.g. a collection of addresses or strings), their life cycle would depend on the parent and no further action would be required for convenient "cascading" of state changes. When the parent is persisted, the value-typed child objects are persisted as well, when the parent is removed, the children will be removed, etc. This even works for operations such as the removal of a child from the collection; Hibernate will detect this and, since value-typed objects can not have shared references, remove the child from the database.

Now consider the same scenario with parent and child objects being entities, not value-types (e.g. categories and items, or parent and child cats). Entities have their own life cycle, support shared references (so removing an entity from the collection does not mean it can be deleted), and there is by default no cascading of state from one entity to any other associated entities. The EJB3 specification does not require persistence by reachability. It supports a more flexible model of transitive persistence, as first seen in Hibernate.

For each basic operation of the entity manager - including persist(), merge(), remove(), refresh() - there is a corresponding cascade style. Respectively, the cascade styles are named PERSIST, MERGE, REMOVE, REFRESH. If you want an operation to be cascaded to associated entity (or collection of entities), you must indicate that in the association annotation:

@OneToOne(cascade=CascadeType.PERSIST)

Cascading options can be combined:

@OneToOne(cascade= { CascadeType.PERSIST, CascadeType.REMOVE, CascadeType.REFRESH } )

You may even use CascadeType.ALL to specify that all operations should be cascaded for a particular association. Remember that by default, no operation is cascaded.

Hibernate offers more native cascading options, please refer to the Hibernate Annotations manual and the Hibernate reference guide for more information.

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