cart.rb

class Cart < ActiveRecord::Base
  has_many :items
  alias_method :items_proxy, :items

  def add_item(options)
    item = items_proxy.build options
    if item.valid?
      item
    else
      items_proxy.delete item
      false
    end
  end

  def items
    @items ||= ReadOnlyProxy.new(items_proxy)
  end
end

cart_spec.rb

describe Cart, "add_item" do
  it "should add the item to the list when it's valid" do
    cart = Cart.new
    cart.add_item :quantity => 5
    cart.should have(1).items
  end

  it "should not add the item when it's not valid" do
    cart = Cart.new
    cart.add_item :quantity => nil
    cart.should have(0).items
  end
end

describe Cart, "items" do
  it "should not allow build or create to be called on it" do
    lambda { Cart.new.items.build :quantity => 5 }.should raise_error(NoMethodError)
  end

  it "should allow typical read methods" do
    cart = Cart.new
    cart.add_item :quantity => 5
    cart.items.first.should_not be_blank
    cart.items.map(&:quantity).should == [5]
  end
end

read_only_proxy.rb

class ReadOnlyProxy
  instance_methods.each { |m| undef_method m unless m =~ /^__/ }
  include Enumerable

  def initialize(proxy)
    @proxy = proxy
  end

  def each(&block)
    @proxy.each &block
  end
end