stubbing sum vs reduce

user.rb

class User < ActiveRecord::Base
  has_many :orders
  has_many :order_items, :through => :orders

  def revenue
    #TODO: stubbing OrderItem#price means I can't use .sum(:price). Is there a better way to use stubs AND sql?
    #order_items.kept.sum(:price)
    order_items.kept.reduce(0) { |sum, n| sum + n.price }
  end

end

user_spec.rb

require "spec_helper"

describe User do
  before(:each) {
    OrderItem.any_instance.stub(:price).and_return(25)
  }

  describe "#revenue" do

    context "no orders" do
      specify { user.revenue.should == 0 }
    end

    context "one order" do
      specify { user_with_no_kept_items.revenue.should == 0 }
      specify { user_with_all_kept_items.revenue.should == 125 }
      specify { user_with_one_kept_item.revenue.should == 25 }
    end

    context "multiple orders" do
      it "should return 50 for two orders, each with one kept item" do
        2.times { FactoryGirl.create :order_with_one_kept_item, user: user }
        user.revenue.should == 50
      end
    end
  
  end