EJB / EE :: JPA Exceptions - How To Handle

In my EJB modules, to prevent that any JPA exception is ever thrown, I check the condition that would cause the exception beforehand. For example, the exception javax.​persistence.EntityExistsException is never thrown because, before persisting the entity, I check if such primary key already exists in the DB. Is it the right way to do this?

Another approach is too allow the JPA exceptions to be thrown and catch them in a try-catch block, and then throw my custom exception from the "catch" block. However it requires to call EntityManager.flush () at the end of the "try" block. Otherwise the exception throw could be deferred and never be caught by my block.