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https://github.com/Shopify/money

Manage money in Shopify with a class that won't lose pennies during division
https://github.com/Shopify/money

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Manage money in Shopify with a class that won't lose pennies during division

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# money



[![tests](https://github.com/Shopify/money/workflows/tests/badge.svg)](https://github.com/Shopify/money/actions?query=workflow%3Atests+branch%3Amain)



`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'



## Upgrading to v1.0



see instructions and breaking changes: https://github.com/Shopify/money/blob/main/UPGRADING.md



## Usage



``` ruby

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`.



``` ruby

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.



``` ruby

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:



``` ruby

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:



```ruby

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).



``` ruby

Money::Currency.new("USD").minor_units  # => 2

Money::Currency.new("JPY").minor_units  # => 0

Money::Currency.new("MGA").minor_units  # => 1

```



## 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:



```ruby

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`

```yaml

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](https://semver.org/).

- Open a PR to update the version accordingly in `lib/money/version`.

- After getting approval, merge the PR.

- [**Publish** a release in Github](https://github.com/Shopify/money/releases/new):

  - 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](https://shipit.shopify.io/shopify/money/production).

- For more information see [the publish a gem vault page](https://vault.shopify.io/page/Publish-a-new-version-of-an-internal-gem~dhbc57d.md)

- You are now responsible to [merge the bump PR in core](https://github.com/Shopify/shopify/pulls?q=is%3Aopen+is%3Apr+author%3Aapp%2Fdependabot+shopify-money+)



## Copyright



Copyright (c) 2011 Shopify. See LICENSE.txt for

further details.