makes_booking_payment.rb

class MakesBookingPayment
  def initialize(booking, payment_details)
    @booking = booking
    @payment_details = payment_details
  end

  def pay!
    response = process_payment
    Transaction.record_purchase(@booking, response)

    if response.success?
      Payment.create_payment(@booking)
    else
      false
    end
  end

  private

  def process_payment
    PaymentProcessor.process(@booking, @payment_details)
  end
end

makes_booking_payment_test.rb

require "minitest/autorun"
require "minitest/pride"
require "mocha/setup"
require "virtus"

require_relative "./makes_booking_payment"

# I'd probably use an actual Booking class if it is simple value object.
class Booking
  include Virtus
  attribute :balance, Integer
end

# I'm also okay with dependency injection, but here it seems easier to
# just stub these classes. When you actually need different payment
# processor, then definitely use DI.
class Transaction; end
class PaymentProcessor; end
class Payment; end

describe MakesBookingPayment do
  # Setup variables for successful path
  let(:booking) { Booking.new(balance: 2.34) }
  let(:payment_details) { mock("Payment details") }
  let(:response) { stub("Payment response", success?: true) }

  let(:action) { MakesBookingPayment.new(booking, payment_details) }

  before do
    Payment.stubs(:create_payment).returns(true)
    Transaction.stubs(:record_purchase)
    PaymentProcessor.stubs(:process).returns(response)
  end

  it "processes the booking using payment processor" do
    PaymentProcessor.
      expects(:process).with(booking, payment_details).returns(response)
    action.pay!
  end

  it "keeps a record of each transaction" do
    Transaction.expects(:record_purchase).with(booking, response)
    action.pay!
  end

  it "creates a payment record when successful transaction" do
    Payment.expects(:create_payment).with(booking)
    action.pay!
  end

  it "returns false when processing payment fails" do
    response.stubs(:success?).returns(false)
    action.pay!.must_equal false
  end
end