Refactored user model

user.rb

class User < ActiveRecord::Base
  ## Omitting Rails stuff...

  attr_accessor :service, :cc_token

  def pay(amount)
    service.pay(amount, external_id: self.external_id)
  end

  def register
    self.external_id = service.register(self.cc_token, name: self.name, email: self.email)
    self.save
  end

  def service
    @service ||= GatewayService
  end
end