Answer by V G for JPA EntityManager: Why use persist() over merge()?

There are some more differences between merge and persist (I will enumerate again those already posted here):

D1. merge does not make the passed entity managed, but rather returns another instance that is managed. persist on the other side will make the passed entity managed:

//MERGE: passedEntity remains unmanaged, but newEntity will be managedEntity newEntity = em.merge(passedEntity);//PERSIST: passedEntity will be managed after thisem.persist(passedEntity);

D2. If you remove an entity and then decide to persist the entity back, you may do that only with persist(), because merge will throw an IllegalArgumentException.

D3. If you decided to take care manually of your IDs (e.g by using UUIDs), then a mergeoperation will trigger subsequent SELECT queries in order to look for existent entities with that ID, while persist may not need those queries.

D4. There are cases when you simply do not trust the code that calls your code, and in order to make sure that no data is updated, but rather is inserted, you must use persist.