I disagree (also see https://www.redhat.com/archives/katello-devel/2013-January/msg00204.html). While there might be a better place for this validator within app/models, it very much describes constraints of Katello model, and therefore belongs in the models dir. I might agree that more generic validators (similar to those provided by Rails, like presence or format validators) could be placed in lib/, but I don't see much point in that (and it's inconvenient).