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 saved, the value-typed child objects are saved as
well, when the parent is deleted, the children will be deleted, 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't have shared
references, delete 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. Hibernate
does not implement persistence by
reachability by default.
For each basic operation of the Hibernate session - including persist(), merge(), saveOrUpdate(), delete(), lock(), refresh(),
evict(), replicate() - there is a corresponding cascade style.
Respectively, the cascade styles are named create, merge,
save-update, delete, lock, refresh, evict, replicate. If you want an
operation to be cascaded along an association, you must indicate that in the
mapping document. For example:
<one-to-one name="person" cascade="persist"/>
Cascade styles my be combined:
<one-to-one name="person" cascade="persist,delete,lock"/>
You may even use cascade="all" to specify that
all operations should be cascaded along the
association. The default cascade="none" specifies
that no operations are to be cascaded.
A special cascade style, delete-orphan, applies
only to one-to-many associations, and indicates that the delete() operation should be applied to any child object
that is removed from the association.
Recommendations:
-
It doesn't usually make sense to enable cascade on a <many-to-one> or <many-to-many> association. Cascade is often useful
for <one-to-one> and <one-to-many> associations.
-
If the child object's lifespan is bounded by the lifespan of the parent
object, make it a life cycle object by
specifying cascade="all,delete-orphan".
-
Otherwise, you might not need cascade at all. But if you think that you will
often be working with the parent and children together in the same transaction,
and you want to save yourself some typing, consider using cascade="persist,merge,save-update".
Mapping an association (either a single valued association, or a collection)
with cascade="all" marks the association as a parent/child style relationship where
save/update/delete of the parent results in save/update/delete of the child or
children.
Futhermore, a mere reference to a child from a persistent parent will result
in save/update of the child. This metaphor is incomplete, however. A child which
becomes unreferenced by its parent is not
automatically deleted, except in the case of a <one-to-many> association mapped with cascade="delete-orphan". The precise semantics of cascading
operations for a parent/child relationship are as follows:
-
If a parent is passed to persist(), all children
are passed to persist()
-
If a parent is passed to merge(), all children are
passed to merge()
-
If a parent is passed to save(), update() or saveOrUpdate(), all
children are passed to saveOrUpdate()
-
If a transient or detached child becomes referenced by a persistent parent,
it is passed to saveOrUpdate()
-
If a parent is deleted, all children are passed to delete()
-
If a child is dereferenced by a persistent parent, nothing special happens - the application should
explicitly delete the child if necessary - unless cascade="delete-orphan", in which case the "orphaned" child
is deleted.
Finally, note that cascading of operations can be applied to an object graph
at call time or at flush time. All operations, if enabled, are
cascaded to associated entities reachable when the operation is executed.
However, save-upate and delete-orphan are transitive for all associated entities
reachable during flush of the Session.