Entitymanager merge not updating record
We will cover cascading of the merge() operation later in this section.If an entity being merged points to a removed entity, an Illegal Argument Exception exception will be thrown.Spitter spitter=new Spitter(); Spittle spittle3=new Spittle(); Username("George"); Password("test1234"); spittle3Spittle("I love java 2"); spittle3Spitter(spitter); Spittle(spittle3); // Spitter spitter=new Spitter(); Spittle spittle3=new Spittle(); Username("George"); Password("test1234"); spittle3Spittle("I love java 2"); spittle3Spitter(spitter); dao.save(spittle3); // Although if connected to My SQL merge() could be as efficient as persist() using a call to INSERT with ON DUPLICATE KEY UPDATE option, JPA is a very high level programming and you can't assume this is going to be the case everywhere.If you're using the assigned generator, using merge instead of persist can cause a redundant SQL statement, therefore affecting performance.Yes, merge() method is used to update the record for the particular entity but it can create new record for associated entities in some situation. User is associated with User Detial with one-to-one mapping.
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Also, calling merge for managed entities is also a mistake since managed entities are automatically managed by Hibernate and their state is synchronized with the database record by the dirty checking mechanism upon flushing the Persistence Context.
To understand how all this works, you should first know that Hibernate shifts the developer mindset from SQL statements to entity state transitions.
If the provider did not update the Employee instance in the persistence context, any references to that instance will become inconsistent with the new state being merged in.
When merge() is invoked on a new entity, it behaves similarly to the persist() operation.
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Persist takes an entity instance, adds it to the context and makes that instance managed (ie future updates to the entity will be tracked).