Using the Uniqueness validator facilitates failure if a matching row already exists, however it seems that duplicate rows could result when using Uniqueness as two coincident requests could both determine that a matching row does not exist and proceed to insert. Is the only solution to use a unique index and rely on insert failure, or is putting a lock, table or row, possible? I'm aware that the unimplemented forUpdate is now available, but is it possible to use explicit LOCK queries?