Why is adding an application-level 'check if it exists first' guard not a real fix for a race-condition-driven ConstraintViolationException?

Two concurrent requests can both pass the application-level existence check before either one has actually committed its insert — the check-then-insert sequence is inherently racy under real concurrency. The actual fix is relying on and gracefully handling the database-level unique/foreign-key constraint itself, since only the database can atomically enforce uniqueness across concurrent transactions.