foo.rb

#!/usr/bin/env ruby

class PropertyObject
  @fields = []

  def self.inherited(descendant)
    descendant.initialize_fields
  end

  def self.initialize_fields
    @fields = []
  end

  def self.fields
    superclass.respond_to?(:fields) ? @fields+superclass.fields : @fields
  end

  def self.add_fields(*fields)
    @fields.concat(fields)
    attr_accessor *fields
  end

  def show
    self.class.fields.each do |e|
      p(__send__(e))
    end
  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, :length, :height, :width, :weight])
ho.show # => should work