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## config/envrionment.rb
config.load_paths += %W(#{RAILS_ROOT}/app/concerns)
(in the Rails::Initializer.run section)

## app/concerns/pennies.rb (create the concerns dir)
module Pennies
  def self.included(base)
    def base.is_pennies(*cols)
      cols.each do |col|
        class_eval "def #{col}_in_dollars; self.#{col} / 100; end"
      end
    end
  end
end

## app/models/unit.rb (and any other class you want)
class Unit < ActiveRecord::Base
  include Pennies
....
  is_pennies :monthly_rent_ammount