Is there anyway to have the validates_attachment_size except a dynamic file size limit? Here's an example:

class Document < ActiveRecord::Base
   belongs_to :folder
   has_attached_file :document
   validates_attachment_size :document, :less_than => get_current_file_size_limit

   private

   def get_current_file_size_limit
     10.megabytes # This will dynamically change
   end
end

I've tried this but I keep getting an error saying "unknown method". Lambdas and Procs don't work either. Has anyone ever tried this? Thanks