What's the best way to test this method?

gistfile1.rb

# Employee Discount model
class Discount::Employee < Discount
  def compute?(order)
    raise "look at MerchandiserStandard.compute?"
    order.front_end? &&
    order.credit_adjustments.empty? && 
    order.discount_adjustments.empty? &&
    order.promotion_credits.empty? &&
    order.has_only_customer_products? &&
    can_use?(self.user)
  end
end

# Employee Discount spec
describe Discount::Employee do
  subject { Discount::Employee.new }

  describe "compute?" do
    let(:order) { mock_model(Order, :credit_adjustments => [], :discount_adjustments => [], :promotion_credits => []) }
    before { subject.stubs(:user) }
    
    it "should call the right methods" do
      order.discount_adjustments.should_receive(:empty?)
      order.should_receive(:front_end?)
      order.promotion_credits.should_receive(:empty?)
      order.should_receive(:has_only_customer_products?)
      subject.should_receive(:can_use?)
      
      subject.compute?(order)
    end
  end
end