describe Item
  describe "#received" do
    it "should return the amount of goods received" do
      receiving_records = mock_model(ReceivingRecord)
      subject.stub(:receiving_records) { receiving_records }
      receiving_records.stub(:sum).with(:quantity) { 3 }
      subject.received.should == 3
    end
  end
end

class Item < ActiveRecord::Base
  has_many :receiving_records

  def received
    receiving_records.sum(:amount)
  end
end