money

money_column expects a DECIMAL(21,3) database field.

Features

Provides a Money class which encapsulates all information about a 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 decimals. No need to convert your amounts every time you use them. Easily understand the data in your DB.
Does NOT provide APIs for exchanging money from one currency to another.
Will not lose pennies during divisions. For instance, given $1 / 3 the resulting chunks will be .34, .33, and .33. Notice that one chunk is larger than the others, so the result still adds to $1.
Allows callers to select a rounding strategy when dividing, to determine the order in which leftover pennies are given out.

Installation

gem 'shopify-money'

Usage

require 'money'

# 10.00 USD
money = Money.new(10.00, "USD")
money.subunits     #=> 1000
money.currency     #=> Money::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(5000, "USD")

m = Money.new(1000, "USD")
# Splitting money evenly
m.split(2)              == [Money.new(500, "USD"), Money.new(500, "USD")]
m.split(3).map(&:value) == [333.34, 333.33, 333.33]
m.calculate_splits(2)   == { Money.new(500, "USD") => 2 }
m.calculate_splits(3)   == { Money.new(333.34, "USD") => 1, Money.new(333.33, "USD") =>2 }

# Allocating money proportionally
m.allocate([0.50, 0.25, 0.25]).map(&:value)               == [500, 250, 250]
m.allocate([Rational(2, 3), Rational(1, 3)]).map(&:value) == [666.67, 333.33]

## Allocating up to a cutoff
m.allocate_max_amounts([500, 300, 200]).map(&:value) == [500, 300, 200]
m.allocate_max_amounts([500, 300, 300]).map(&:value) == [454.55, 272.73, 272.72]

## Selectable rounding strategies during division

# Assigns leftover subunits left to right
m = Money::Allocator.new(Money.new(10.55, "USD"))
monies = m.allocate([0.25, 0.5, 0.25], :roundrobin)
#monies[0] == 2.64  <-- gets 1 penny
#monies[1] == 5.28  <-- gets 1 penny
#monies[2] == 2.63  <-- gets no penny

# Assigns leftover subunits right to left
m = Money::Allocator.new(Money.new(10.55, "USD"))
monies = m.allocate([0.25, 0.5, 0.25], :roundrobin_reverse)
#monies[0] == 2.63  <-- gets no penny
#monies[1] == 5.28  <-- gets 1 penny
#monies[2] == 2.64  <-- gets 1 penny

# Assigns leftover subunits to the nearest whole subunit
m = Money::Allocator.new(Money.new(10.55, "USD"))
monies = m.allocate([0.25, 0.5, 0.25], :nearest)
#monies[0] == 2.64  <-- gets 1 penny
#monies[1] == 5.27  <-- gets no penny
#monies[2] == 2.64  <-- gets 1 penny
# $2.6375 is closer to the next whole penny than $5.275

# Clamp
Money.new(50, "USD").clamp(1, 100) == Money.new(50, "USD")

# Unit to subunit conversions
Money.from_subunits(500, "USD")  == Money.new(5, "USD")   # 5 USD
Money.from_subunits(5, "JPY")    == Money.new(5, "JPY")   # 5 JPY
Money.from_subunits(5000, "TND") == Money.new(5, "TND")   # 5 TND

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(1000, Money::Currency.new("USD"))
Money.new(1000, "EUR").currency == Money::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"
currency.to_s     #=> 'USD'
currency.symbol   #=> '$'
currency.disambiguate_symbol #=> 'US$'

Default Currency

By default Money defaults to Money::NullCurrency as its currency. This is a global variable that can be changed using:

Money.configure do |config|
  config.default_currency = Money::Currency.new("USD")
end

In web apps you might want to set the default currency on a per request basis. In Rails you can do this with an around action, for example:

class ApplicationController < ActionController::Base
  around_action :set_currency

  private

  def set_currency
    Money.with_currency(current_shop.currency) { yield }
  end
end

Currency Minor Units

The exponent of a money value is the number of digits after the decimal separator (which separates the major unit from the minor unit).

Money::Currency.new("USD").minor_units  # => 2
Money::Currency.new("JPY").minor_units  # => 0
Money::Currency.new("MGA").minor_units  # => 1

Convert Currency

Money.new(money * exchange_rate, "JPY") will raise an exception. The valid alternatives are:

Money.new(money.value * exchange_rate, "JPY")
# Or
money.convert_currency(exchange_rate, "JPY")

Money column

Since money internally uses BigDecimal it's logical to use a decimal column for your database. The money_column method can generate methods for use with ActiveRecord:

create_table :orders do |t|
  t.decimal :sub_total, precision: 21, scale: 3
  t.decimal :tax, precision: 21, scale: 3
  t.string :currency, limit: 3
end

class Order < ApplicationRecord
  money_column :sub_total, :tax
end

Options

option	type	description
currency_column	method	column from which to read/write the currency
currency	string	hardcoded currency value
currency_read_only	boolean	when true, `currency_column` won't write the currency back into the db. Must be set to true if `currency_column` is an attr_reader or delegate. Default: false
coerce_null	boolean	when true, a nil value will be returned as Money.zero. Default: false

You can use multiple money_column calls to achieve the desired effects with currency on the model or attribute level.

There are no validations generated. You can add these for the specified money and currency attributes as you normally would for any other.

Rubocop

A RuboCop rule to enforce the presence of a currency using static analysis is available.

Add to your .rubocop.yml

require:
  - money

Money/MissingCurrency:
  Enabled: true
  # ReplacementCurrency: CAD

Money/ZeroMoney:
  Enabled: true
  # ReplacementCurrency: CAD

Contributing to money

Check out the latest main to make sure the feature hasn't been implemented or the bug hasn't been fixed yet
Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it
Fork the project
Start a feature/bugfix branch
Commit and push until you are happy with your contribution
Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.

Releasing

To release a new version of the gem, follow these steps:

Audit what has changed since the last version of the gem was released.
Determine what the next version number should be, according to Semantic Versioning.
Open a PR to update the version accordingly in lib/money/version.
After getting approval, merge the PR.
Publish a release in Github:
- Target the main branch with a tag matching the new version, prefixed with v (e.g. v1.2.3).
- Use the "Generate Release Notes" button to help generate the copy for the release. Include consumer facing changes in the release notes.
Deploy the new version to Rubygems using ShipIt.
For more information see the publish a gem vault page
You are now responsible to merge the bump PR in core

Shopify/money