has_money.rb

module HasMoney
  
  def self.included(base)
    base.extend ClassMethods
  end
  
  module ClassMethods
    
    # 
    # has_money :default_price
    # 
    # def default_price_in_dollars
    # end
    # 
    # def default_price_in_dollars=(dollars)
    # end
    
    def has_money(*attributes)
      attributes.each do |attribute|
        class_eval <<-EOS      
          def #{attribute}_in_dollars
            self.class.calculate_dollars_from_cents(#{attribute})
          end
      
          def #{attribute}_in_dollars=(dollars)
            self.#{attribute} = self.class.calculate_cents_from_dollars(dollars)
          end
        EOS
      end
    end
    
    def calculate_dollars_from_cents(cents)
      return nil if cents.nil? ||  cents == ''
      cents.to_f / 100
    end

    def calculate_cents_from_dollars(dollars)
      return nil if dollars.nil? || dollars == ''
      (dollars.gsub(/[^0]/).to_f * 100).round
    end
    
  end
end

# Inlcude in all AR Models
ActiveRecord::Base.send :include, HasMoney

has_money_spec.rb

require File.expand_path(File.dirname(__FILE__) + '/../spec_helper')

describe HasMoney do
  
  describe "including has money" do
    it "should extend the class with class methods when included" do
      klass = Class.new
      klass.send :include, HasMoney
      klass.should respond_to(:has_money)
    end
  end
  
  describe "working with money" do
    before(:each) do
      @klass = Class.new
      @klass.class_eval do
        include HasMoney
        attr_accessor :price
        has_money :price
      end
      @product = @klass.new
      @product.price = 2999
    end
    
    it "should create an accessor method named in_dollars for the attribute" do
      @product.should respond_to(:price_in_dollars)
    end
    
    it "should create a setter method named in_dollars for the attribute" do
      @product.should respond_to(:price_in_dollars=)
    end

    it "should return the amount in dollars" do
      @product.price_in_dollars.should == 29.99
    end
    
    it "should set the amount in dollars with a float" do
      @product.price_in_dollars = 39.99
      @product.price.should == 3999
    end
    
    it "should set the amount in dollars with a string" do
      @product.price_in_dollars = '39.99'
      @product.price.should == 3999
    end
    
    it "should round the amount up to the nearest cent" do
      @product.price_in_dollars = '39.998'
      @product.price.should == 4000
    end

    it "should round the amount down to the nearest cent" do
      @product.price_in_dollars = '39.992'
      @product.price.should == 3999
    end
    
    it "should set nil if nil is passed as dollars" do
      @product.price_in_dollars = nil
      @product.price.should == nil
    end
    
    it "should set nil if an empty string is passed in as dollars" do
      @product.price_in_dollars = ''
      @product.price.should == nil      
    end
    
    it "should return nil if the attribute is nil" do
      @product.price = nil
      @product.price_in_dollars.should == nil
    end
  end
  
end