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Hibernate update vs merge

May 7, 2012
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Update and Merge both work on detached instances,
ie instances which have a corresponding entry in the DB but which are
currently not attached to (or managed by) a Session. The difference between
them are what happens to the instance which is passed to the function.
update tries to reattach the instance, that means that there may be no
other instance of the persistent entity attached to the Session right now otherwise
a exception is thrown. merge, however, just copies all values to a persistent instance in the Session
(which will be loaded if it is not currently loaded).
The input object is not changed. So merge is more general than update, but may use more resources

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