Answer by Peter Willems for JPA EntityManager: Why use persist() over merge()?

You may have come here for advice on when to use persist and when to use merge. I think that it depends the situation: how likely is it that you need to create a new record and how hard is it to retrieve persisted data.

Let's presume you can use a natural key/identifier.

• Data needs to be persisted, but once in a while a record exists and an update is called for. In this case you could try a persist and if it throws an EntityExistsException, you look it up and combine the data:

  try { entityManager.persist(entity) }

  catch(EntityExistsException exception) { /* retrieve and merge */ }

• Persisted data needs to be updated, but once in a while there is no record for the data yet. In this case you look it up, and do a persist if the entity is missing:

  entity = entityManager.find(key);

  if (entity == null) { entityManager.persist(entity); }

  else { /* merge */ }

If you don't have natural key/identifier, you'll have a harder time to figure out whether the entity exist or not, or how to look it up.

The merges can be dealt with in two ways, too:

1. If the changes are usually small, apply them to the managed entity.
2. If changes are common, copy the ID from the persisted entity, as well as unaltered data. Then call EntityManager::merge() to replace the old content.