foo.rb

#!/usr/bin/env ruby
require 'active_support/core_ext/class/attribute'

class PropertyObject
  class_attribute :fields
  self.fields = []
  def self.add_fields *f
    self.fields += f
    attr_accessor *f
  end

  def show
    self.class.fields.each{|e| p send(e)}
  end
end

class PhysicalObject < PropertyObject
  add_fields :length, :height, :width
end

class HeavyObject < PhysicalObject
  add_fields :weight
end

ho = HeavyObject.new
ho.length = 1
ho.height = 2
ho.width = 3
ho.weight = 55
p PhysicalObject.fields # => [:length, :height, :width]  (which is correct)
p HeavyObject.fields # => [:weight]  (want [:length, :height, :width, :weight])
ho.show # => only "55", of course, because of ☝

(BTW, this is actually working, those comments at the bottom are out of date)