21.4. Cascades and unsaved-value
Suppose we loaded up a
Parent
in one Session
, made some changes in a UI action and wanted to persist these changes in a new session by calling update()
. The Parent
will contain a collection of children and, since the cascading update is enabled, Hibernate needs to know which children are newly instantiated and which represent existing rows in the database. We will also assume that both Parent
and Child
have generated identifier properties of type Long
. Hibernate will use the identifier and version/timestamp property value to determine which of the children are new. (See Section 10.7, “Automatic state detection”.) In Hibernate3, it is no longer necessary to specify an unsaved-value
explicitly.
The following code will update
parent
and child
and insert newChild
:
//parent and child were both loaded in a previous session parent.addChild(child); Child newChild = new Child(); parent.addChild(newChild); session.update(parent); session.flush();
This may be suitable for the case of a generated identifier, but what about assigned identifiers and composite identifiers? This is more difficult, since Hibernate cannot use the identifier property to distinguish between a newly instantiated object, with an identifier assigned by the user, and an object loaded in a previous session. In this case, Hibernate will either use the timestamp or version property, or will actually query the second-level cache or, worst case, the database, to see if the row exists.