Persistence by reachability is the idea that if a transient object A references another transient object B, then persisting A will also persist B.
Both Naked Objects and also JPA support this
concept. The Naked Objects' support is through the
PersistAlgorithm interface. This allows object
store implementations to instruct the persistor as to how to traverse
the object graph. For example, the default algorithm (used by the
in-memory object store) is implemented in
DefaultPersistAlgorithm. This queues objects for
persisting bottom-up, referred to (child objects) first and the parent
objects. The TopDownAlgorithm, on the other hand,
works the other way. The ability to plug in different algorithms
provides flexibility for different persistence technologies.
JPA Objects uses (a trivial subclass of) the
DefaultPersistAlgorithm. This is sufficient
because the JPA provider (Hibernate) itself will persist objects in the
correct order.
If required, you can also set the
CascadeType using the cascade attribute of
@ManyToOne and @OneToOne
annotations. This can be thought of as a generalization of the
persistence-by-reachability idea, cascading not only persists of
transient objects, but also of updates and of removals
(deletions).
CascadeType can take the following
values:
PERSIST
Cascades calls to
EntityManager#persist(), in other words,
persistence-by-reachability for transient instances referenced
through the specified association.
MERGE
Cascades calls to
EntityManager#merge(). This is equivalent
to cascading updates .
REMOVE
Cascades calls to
EntityManager#remove(). This is equivalent
to cascading deletions.
REFRESH
Cascades calls to
EntityManager#refresh(). This is equivalent
to cascading refreshes (reloading an instance after a database
change, for example a trigger running).
ALL
Shortcut for cascade={PERSIST, MERGE, REMOVE,
REFRESH}
In a Naked Objects application these methods are called from
JPA Objects, not from your domain objects.
specicailly, in the implementations of
PersistenceCommand (for example
JpaCreateCommand). Even so, there may be
occasions when these annotations will influence the normal behaviour to
accomplish a specific objective.
Over time a set of patterns may emerge to help define suitable CascadeType settings for various domain model relationships. If so, this guide will be updated to provide further guidance.