cart.rb

class Cart
  include MongoMapper::Document

  many :cart_items
  timestamps!
  
  def add_new_item_or_increase_quantity(product_id, quantity)
    if self.has_product?(product_id)
      self.increment_cart_item_quantity(product_id, quantity)
    else
      self.add_new_cart_item(product_id, quantity)
    end
  end

  def add_new_cart_item(product_id, quantity)
    cart_item = CartItem.new(:product_id => product_id, :quantity => quantity)
    self.cart_items << cart_item
  end
  
  def increment_cart_item_quantity(product_id, quantity)
    cart_item = find_existing_cart_item(product_id)
    cart_item.quantity += quantity.to_i
    cart_item.save
  end

  def has_product?(product_id)
    if find_existing_cart_item(product_id)
      true
    else
      false
    end
  end

  def find_existing_cart_item(product_id)
    self.cart_items.select { |ci| ci.product_id == product_id.to_i }.first
  end

end

cart_item.rb

class CartItem
  include MongoMapper::Document

  belongs_to :cart
  one :product
  key :quantity
  key :cart_id, ObjectId
end

product.rb

class Product
  include MongoMapper::EmbeddedDocument

  key :name, String
  key :description, String
  key :cart_item_id, ObjectId
end