Example utility class for running a transaction

debit.rb

module Banking
  module Processing
    class Debit
      class << self 
        def execute(amount, payment_method)
          # Do vital checks before doing any work
          unless payment_method.registered?
            raise Errors::UnregisteredPaymentMethod.new("You can't debit an unregistered payment method")
          end
          
          # Hit the API 
          SomeGateway::API.sale(amount: amount, payment_method)
          
          # Manipulate local data
          transaction = Transaction.create gateway_response.merge(amount: amount)
          payment_method.transactions << transaction
          transaction
        end
      end
    end
  end
end