line_item.rb

class LineItem < ActiveRecord::Base

  belongs_to :invoice
  
  def allocated?
    status == 'allocated'
  end
  
  def refundable?
    allocated? && invoice.refundable?
  end

  def cancellable?
    allocated? && invoice.cancellable?
  end
end

line_item_spec.rb

describe LineItem do
  describe "permissions" do

    subject do
      LineItem.new(status: status, invoice: stub_model(Invoice, invoice_attr))
    end

    context "with an invoice that allows for refunds and cancellations" do
      let(:invoice_attr) { {refundable?: true, cancellable?: true} }

      context "and allocated" do
        let(:status) { "allocated" }
        it { should be_refundable }
        it { should be_cancellable }
      end

      context "and not allocated" do
        let(:status) { "cancelled" }
        it { should_not be_refundable }
        it { should_not be_cancellable }
      end
    end

    context "with an invoice that doesn't allow for refunds and cancellations" do
      let(:invoice_attr) { {refundable?: false, cancellable?: false} }
      let(:status) { "allocated" }
      it { should_not be_refundable }
      it { should_not be_cancellable }
    end

  end
end