gistfile1.txt

  Units = { :dollar_per_ffe => { :symbol => "$/ffe", :si_factor=>1.0 },
            :dollar_per_ton => { :symbol => "$/T", :si_factor=>1.0e-3 }
          }

  Attribute_Units = { :default_transshipment_cost => :dollar_per_ffe,
                      :bunker_cost => :dollar_per_ton
                    }

  def bunker_cost
    read_attribute(:bunker_cost) / Units[Attribute_Units[:bunker_cost]][:si_factor];
  end

  def bunker_cost=(value)
    write_attribute(:bunker_cost, value * Units[Attribute_Units[:bunker_cost]][:si_factor]);
  end

  def default_transshipment_cost
     # as bunker_cost
  end