cart.rb

class Cart < ActiveRecord::Base
  has_many :items, :class_name => 'CartItem', :dependent => :destroy
  
  # ...
  
  def empty!
    items.each(&:destroy)
    self
  end
  
end

cart_spec.rb

describe Cart, "with some items" do  
  before do
    @cart = Factory(:cart)
    @cart.items << CartItem.new(:price => 10.0, :quantity => 2)
    @cart.items << CartItem.new(:price => 10.0, :quantity => 1)
  end

  # ...
  
  it "can empty itself" do
    @cart.empty!
    @cart.item_count.should == 0
  end
end

# Fails:
# 'Cart with some items can empty itself' FAILED
# expected: 0,
#      got: 3 (using ==)