wallet_central.rb

#!/usr/bin/env ruby

require 'sequel'
require 'json'
require './lib/currency'

DB = Sequel.connect('sqlite://db/wallets.db')

class WalletCentral
  def self.output(name=nil)
    if name.nil?
      DB[:wallets].select_hash_groups(:name, [:currency, :amount]).to_json
    else
      DB[:wallets].where(name: name).select_hash_groups(:name, [:currency, :amount]).to_json
    end
  end

  def self.transfer(from, to, currency, target_currency=nil, amount)
    sender_currency = DB[:wallets].where(name: from, currency: currency).get(:currency)
    recipient_currencies = DB[:wallets].where(name: to).select_map(:currency)

    raise 'No such currency for sender' if sender_currency.nil?

    if recipient_currencies.include?(sender_currency) && target_currency.nil?
      DB.transaction do
        DB[:wallets].where(name: from, currency: currency).update(amount: Sequel[:amount]-amount)
        DB[:wallets].where(name: to, currency: currency).update(amount: Sequel[:amount]+amount)
      end
    else
      price = Currency.convert(amount, target_currency)

      DB.transaction do
        DB[:wallets].where(name: from, currency: currency).update(amount: Sequel[:amount]-amount)
        DB[:wallets].where(name: to, currency: target_currency).update(amount: Sequel[:amount]+price)
      end
    end
  end
end

WalletCentral.transfer('jon', 'sansa', 'USD', 'BRL', 1)
puts WalletCentral.output