Some more details about merge which will help you to use merge over persist:
Returning a managed instance other than the original entity is a critical part of the merge process. If an entity instance with the same identifier already exists in the persistence context, the provider will overwrite its state with the state of the entity that is being merged, but the managed version that existed already must be returned to the client so that it can be used. 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. It adds the entity to the persistence context, but instead of adding the original entity instance, it creates a new copy and manages that instance instead. The copy that is created by the merge() operation is persisted as if the persist() method were invoked on it.
In the presence of relationships, the merge() operation will attempt to update the managed entity to point to managed versions of the entities referenced by the detached entity. If the entity has a relationship to an object that has no persistent identity, the outcome of the merge operation is undefined. Some providers might allow the managed copy to point to the non-persistent object, whereas others might throw an exception immediately. The merge() operation can be optionally cascaded in these cases to prevent an exception from occurring. We will cover cascading of the merge() operation later in this section. If an entity being merged points to a removed entity, an IllegalArgumentException exception will be thrown.
Lazy-loading relationships are a special case in the merge operation. If a lazy-loading relationship was not triggered on an entity before it became detached, that relationship will be ignored when the entity is merged. If the relationship was triggered while managed and then set to null while the entity was detached, the managed version of the entity will likewise have the relationship cleared during the merge."
All of the above information was taken from "Pro JPA 2 Mastering the Java™ Persistence API" by Mike Keith and Merrick Schnicariol. Chapter 6. Section detachment and merging. This book is actually a second book devoted to JPA by authors. This new book has many new information then former one. I really recommed to read this book for ones who will be seriously involved with JPA. I am sorry for anonimously posting my first answer.