The merge operation is clever enough to automatically detect whether the merging of the detached instance has to result in an insert or update. In other words, you don't have to worry about passing a new instance (and not a detached instance) to merge(), the entity manager will figure this out for you:

// In the first entity manager
Cat cat = firstEntityManager.find(Cat.class, catID);

// In a higher layer of the application, detached
Cat mate = new Cat();
cat.setMate(mate);

// Later, in a new entity manager
secondEntityManager.merge(cat);   // update existing state
secondEntityManager.merge(mate);  // save the new instance

The usage and semantics of merge() seems to be confusing for new users. Firstly, as long as you are not trying to use object state loaded in one entity manager in another new entity manager, you should not need to use merge() at all. Some whole applications will never use this method.

Usually merge() is used in the following scenario:

Here is the exact semantic of merge():