wallet_central.rb

#!/usr/bin/env ruby

require 'sequel'
require 'json'

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, amount)
    from_amount = DB[:wallets].where(name: from, currency: currency).get(:amount)

    if from_amount >= 0
      DB[:wallets].where(name: from, currency: currency).update(amount: Sequel[:amount]-amount)
      DB[:wallets].where(name: to, currency: currency).update(amount: Sequel[:amount]+amount)
    else
      raise "Amount insufficient"
    end
  end
end

puts WalletCentral.output
WalletCentral.transfer('jon', 'jaime', 'USD', 100.0)