Question

Does anyone know how Hibernate knows whether to INSERT or to UPDATE a value in the database when session.saveOrUpdate() is called?

So far, I have only determined that it is not dependent on the information in the cache, and that the existence of the entity in the database is determined by the primary key.

Answer 1

When you use .saveOrUpdate() Hibernate will check if the object is transient (it has no identifier property) and if so it will make it persistent by generating it the identifier and assigning it to session. If the object has an identifier already it will perform .update().

From the documentation:

saveOrUpdate() does the following:

• if the object is already persistent
  in this session, do nothing
• if another object associated with the
  session has the same identifier,
  throw an exception
• if the object has no identifier
  property, save() it
• if the object’s identifier has the
  value assigned to a newly
  instantiated object, save() it
• if the object is versioned by a
  “version” or “timestamp”, and the
  version property value is the same
  value assigned to a newly
  instantiated object, save() it
  otherwise update() the object