cart-gist.rb

class Cart
  has_many :cart_items
  
  def add_item item_id, quantity = 1
   cart_item = cart_items.find_by_item_id(item_id) || cart_items.create :item_id => item_id
   cart_item.update_attributes :quantity => cart_item.quantity + quantity
  end
end

class CartItem
  belongs_to :cart

  validates_uniqueness_of :item_id, :scope => :cart_id
  
  # or just set defualt => 0 in your schema
  def quantity
    read_attribute(:quantity) || 0
  end
end

class ApplicationController
  before_filter :cart

  protected

  def cart
    @cart = Cart.find( session[:cart_id] ) if session[:cart_id].present?
    @cart ||= Cart.create :ip_address => request.ip_address

    session[:cart_id] = @cart.id

    @cart
  end
end

class CartItemsController
  def new
    @cart.add_item(params)
    redirect_to request.referrer
  end
end