dm_attribute_set.rb

# dm modle
class SampleOrder
  include DataMapper::Resource
  property :id, Serial
  property :order_no, String
  ...

  before :save, :set_order_no

  def set_order_no 
    attribute_set(:order_no, next_order_no) if order_no.nil? || order_no.blank?
  end
  
  private
    def next_order_no
      ...
    end
end