This library aids one in handling money and different currencies. Features:

• Provides a Money class which encapsulates all information about an certain amount of money, such as its value and its currency.

• Represents monetary values as integers, in cents. This avoids floating point rounding errors.

• Provides APIs for exchanging money from one currency to another.

• Has the ability to parse a money string into a Money object.

Resources:

• Website: money.rubyforge.org

• RDoc API: money.rubyforge.org

• Git repository: github.com/FooBarWidget/money/tree/master

Install stable releases with the following command:

gem install money

The development version (hosted on Github) can be installed with:

gem sources -a https://gems.github.com
gem install FooBarWidget-money

require 'money'

# 10.00 USD
money = Money.new(1000, "USD")
money.cents     # => 1000
money.currency  # => "USD"

Money.new(1000, "USD") == Money.new(1000, "USD")   # => true
Money.new(1000, "USD") == Money.new(100, "USD")    # => false
Money.new(1000, "USD") == Money.new(1000, "EUR")   # => false

Exchanging money is performed through an exchange bank object. The default exchange bank object requires one to manually specify the exchange rate. Here’s an example of how it works:

Money.add_rate("USD", "CAD", 1.24515)
Money.add_rate("CAD", "USD", 0.803115)

Money.us_dollar(100).exchange_to("CAD")  # => Money.new(124, "CAD")
Money.ca_dollar(100).exchange_to("USD")  # => Money.new(80, "USD")

Comparison and arithmetic operations work as expected:

Money.new(1000, "USD") <=> Money.new(900, "USD")   # => 1; 9.00 USD is smaller
Money.new(1000, "EUR") + Money.new(10, "EUR") == Money.new(1010, "EUR")

Money.add_rate("USD", "EUR", 0.5)
Money.new(1000, "EUR") + Money.new(1000, "USD") == Money.new(1500, "EUR")

There is nothing stopping you from creating bank objects which scrapes www.xe.com for the current rates or just returns rand(2):

Money.default_bank = ExchangeBankWhichScrapesXeDotCom.new

Use the compose_of helper to let Active Record deal with embedding the money object in your models. The following example requires a cents and a currency field.

class ProductUnit < ActiveRecord::Base
  belongs_to :product
  composed_of :price, :class_name => "Money", :mapping => [%w(cents cents), %w(currency currency)]

  private        
    validate :cents_not_zero

    def cents_not_zero
      errors.add("cents", "cannot be zero or less") unless cents > 0
    end

    validates_presence_of :sku, :currency
    validates_uniqueness_of :sku        
end

By default Money defaults to USD as its currency. This can be overwritten using

Money.default_currency = "CAD"

If you use Rails, then environment.rb is a very good place to put this.