def ensureUserNotExist(cls, **args): return ViewUtils.ensureObjectNotExist( User, ErrorType.UsernameNotExist, **args)
def ensureFriendNotExist(cls, **args): return ViewUtils.ensureObjectNotExist( Friend, ErrorType.AlreadyAFriend, **args)
def ensureParticipationNotExist(cls, uid, pid): return ViewUtils.ensureObjectNotExist( Participation, ErrorType.HaveParticipated, user_id=uid, project_id=pid)