RubyMoney - Money

Introduction

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.
Provides a Money::Currency class which encapsulates all information about a monetary unit.
Represents monetary values as integers, in cents. This avoids floating point rounding errors.
Represents currency as Money::Currency instances providing an high level of flexibility.
Provides APIs for exchanging money from one currency to another.
Has the ability to parse a money and currency strings into the corresponding Money/Currency object.

Resources

Downloading

Install stable releases with the following command:

gem install money

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

git clone git://github.com/RubyMoney/money.git
cd money
rake install

Usage

require 'money'

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

# Comparisons
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
Money.new(1000, "USD") != Money.new(1000, "EUR")   #=> true

# Arithmetic
Money.new(1000, "USD") + Money.new(500, "USD") == Money.new(1500, "USD")
Money.new(1000, "USD") - Money.new(200, "USD") == Money.new(800, "USD")
Money.new(1000, "USD") / 5                     == Money.new(200, "USD")
Money.new(1000, "USD") * 5                     == Money.new(5000, "USD")

# Currency conversions
some_code_to_setup_exchange_rates
Money.new(1000, "USD").exchange_to("EUR") == Money.new(some_value, "EUR")

Currency

Currencies are consistently represented as instances of Money::Currency. The most part of Money APIs allows you to supply either a String or a Money::Currency.

Money.new(1000, "USD") == Money.new(900, Currency.new("USD"))
Money.new(1000, "EUR").currency == Currency.new("EUR")

A Money::Currency instance holds all the information about the currency, including the currency symbol, name and much more.

currency = Money.new(1000, "USD").currency
currency.iso_code #=> "USD"
currency.name     #=> "United States Dollar"

To define a new Money::Currency simply add a new item to the Money::Currency::TABLE hash, where the key is the identifier for the currency object and the value is a hash containing all the currency attributes.

 Money::Currency::TABLE[:usd] = {
  :priority        => 1,
  :iso_code        => "USD",
  :name            => "United States Dollar",
  :symbol          => "$",
  :subunit         => "Cent"
  :subunit_to_unit => 100,
  :separator       => ".",
  :delimiter       => ","
}

The pre-defined set of attributes includes:

:priority a numerical value you can use to sort/group the currency list
:iso_code the international 3-letter code as defined by the ISO 4217 standard
:name the currency name
:symbol the currency symbol (UTF-8 encoded)
:subunit the name of the fractional monetary unit
:subunit_to_unit the proportion between the unit and the subunit
:separator character between the whole and fraction amounts
:delimiter character between each thousands place

All attributes are optional. Some attributes, such as :symbol, are used by the Money class to print out a representation of the object. Other attributes, such as :name or :priority, exist to provide a basic API you can take advantage of to build your application.

:priority

The priority attribute is an arbitrary numerical value you can assign to the Money::Currency and use in sorting/grouping operation.

For instance, let's assume your Rails application needs to render a currency selector like the one available here. You can create a couple of custom methods to return the list of major currencies and all currencies as follows:

# Returns an array of currency id where
# priority < 10
def major_currencies(hash)
  hash.inject([]) do |array, (id, attributes)|
    priority = attributes[:priority]
    if priority && priority < 10
      array[priority] ||= []
      array[priority] << id
    end
    array
  end.compact.flatten
end

# Returns an array of all currency id
def all_currencies(hash)
  hash.keys
end

major_currencies(Money::Currency::TABLE)
# => [ :usd, :eur, :bgp, :cad ]

all_currencies(Money::Currency::TABLE)
# => [ :aed, :afn, all, ... ]

Default Currency

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

Money.default_currency = Money::Currency.new("CAD")

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

Currency Exchange

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 XE for the current rates or just returns rand(2):

Money.default_bank = ExchangeBankWhichScrapesXeDotCom.new

Implementations

The following is a list of Money.gem compatible currency exchange rate implementations.

Ruby on Rails

Use the compose_of helper to let Active Record deal with embedding the money object in your models. The following example requires 2 columns:

:price_cents, :integer, :default => 0, :null => false
:currency, :string

Then in your model file:

composed_of :price,
  :class_name => "Money",
  :mapping => [%w(price_cents cents), %w(currency currency_as_string)],
  :constructor => Proc.new { |cents, currency| Money.new(cents || 0, currency || Money.default_currency) },
  :converter => Proc.new { |value| value.respond_to?(:to_money) ? value.to_money : raise(ArgumentError, "Can't convert #{value.class} to Money") }

For Money 2.2.x and previous versions, simply use the following composed_of definition:

composed_of :price,
  :class_name => "Money",
  :mapping => [%w(cents cents), %w(currency currency)],
  :constructor => Proc.new { |cents, currency| Money.new(cents || 0, currency || Money.default_currency) }

For further details read the full discussion here.

codebrew/money