No EntityManager with actual transaction available for current thread - cannot reliably process 'persist' call

Repository 를 통해 DB에 Save시,

No EntityManager with actual transaction available for current thread - cannot reliably process 'persist' call

와 같은 에러가 뜬다면, save 부분에 @Transactional 어노테이션을 빼먹은 것이다.
해당 메소드에 @Transactional을 선언해주자.