Hmm, I'm not sure I follow how it makes the code clearer to be checking for a case that can't happen. When I saw `blank?` I puzzled over how it could be `blank?` but not `nil?`.
Anyway, there should also be a unique index on this pair of fields in Mongo. That will make it fast if you need to look them up, and act as a safety net if your Rails code were to have a bug and let duplicates through.