Try the "if" option, mentioned here: http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#M000816
So you could, for example, add one of
:if => "!canceled" :if => Proc.new {|invitee| !invitiee.canceled } :if => not_canceled? # where not_canceled is a method in your Invitee class