Money Historical Bank

A gem that add a Money::Bank able to handle historical rates, and infer rates from limited rates.

• You can add rates for any currency pair, at a special date. Use home_run gem if you need fast date handling in ruby.
• If no rates exist for a given date, the gem will try to download historical rates from open-exchange-rates
• The gem is able to guess inverse rates (EURUSD rate when only USDEUR is present), and go through USD when using other pairs. For example, GBPEUR will be calculated using USDGBP and USDEUR. This strategy isn’t used if said rate (GBPEUR) is already set.
• No caching exists, but you can use import_data and export_data the same way Money::Bank::VariableExchange works.

Usage

require 'money/bank/historical_bank'
mh = Money::Bank::HistoricalBank.new

# Exchanges 1000 EUR to USD using Date.today (default if no date has been entered).
# Will download today's rates if none have been entered
mh.exchange_with(1000.to_money('EUR'), 'USD')

# Exchanges 1000 EUR to USD using historical rates
date = Date.new(2009,9,9)
mh.set_rate(date, 'USD', 'EUR', 0.7634)
mh.exchange_with(date, 1000.to_money('USD'), 'EUR') # => 763.4 EUR

Money.default_bank = mh

Using with ActiveRecord

You can use any persistent store that implements the add_rate(date, from, to, rate) and get_rates(date) methods. Here is an example implementation using an ActiveRecord model:

First, generate a model:

rails g model exchange_rate from:string to:string rate:float date:date

Then, implement the store interface in the model:

# app/models/exchange_rate.rb
class ExchangeRate < ApplicationRecord
  # Add new exchange rate.
  # @param [String] from Currency ISO code. ex. 'USD'
  # @param [String] to Currency ISO code. ex. 'CAD'
  # @param [Numeric] rate Exchange rate. ex. 0.0016
  # @param [Date] date Exchange rate date. ex. Date.today
  def self.add_rate(from, to, rate, date)
    exrate = find_or_initialize_by(from: from, to: to, date: date)
    exrate.rate = rate
    exrate.save!
  end

  # Get rate.
  # @param [String] from Currency ISO code. ex. 'USD'
  # @param [String] to Currency ISO code. ex. 'CAD'
  # @param [Date] date Exchange rate date. ex. Date.today
  #
  # @return [Numeric] rate.
  def self.get_rate(from, to, date)
    rate = find_by(from:, to:, date:)
    rate&.rate
  end

  # Gets all rates for a given date.
  #
  # @param [Date] date The date to retrieve rates for.
  # @return [Hash] A hash of rates for the given date.
  def self.get_rates(date)
    where(date: date).each_with_object({}) do |rate, hash|
      key = "#{rate.from}_TO_#{rate.to}".upcase
      hash[key] = rate.rate
    end
  end

  def self.marshal_dump
    [self]
  end
end

Finally, initialize the bank with your new store:

# config/initializers/money.rb
require 'money/bank/historical_bank'

# Use the ExchangeRate model as the store
store = ExchangeRate
Money.default_bank = Money::Bank::HistoricalBank.new(store)

Refs

Created using mainly the base VariableExchange implementation, OpenExchangeRates implementation and idea based on money-open-exchange-rates gem.

• https://github.com/currencybot/open-exchange-rates
• https://github.com/RubyMoney/money
• https://github.com/spk/money-open-exchange-rates

Extension

Feel free to create a new loader (cf OpenExchangeRatesLoader) if you know a source for more historical data.
Feel free to suggest refactoring.

Releasing

This project is released by Travis when tags are pushed and one can do this like
so:

$ ./bin/release 0.0.4 0.0.5

So that command will bump the gem to version 0.0.5, push on master, push tags
and trigger Travis to release.

Note: this script assumes you have called your remote origin, that it’s
pointing at atwam/money-historical-bank and that you have push access.