def add(product_object, amount=nil, unit=nil)
      product_object = (product_object)
      product_object.amount = amount if amount
      product_object.unit = unit if unit

      cart_item_params = product_object.to_cart_item_params
      existing_item = find_item(cart_item_params)

      if existing_item
        existing_item.inc(amount: cart_item_params[:amount])
      else
        cart_items.create!(cart_item_params)
      end
    end
def add(product_object, amount=nil, unit=nil)
      product_object = (product_object)
      product_object.amount = amount if amount
      product_object.unit = unit if unit

      cart_item_params = product_object.to_cart_item_params
      existing_item = find_item(cart_item_params)

      if existing_item
        existing_item.inc(amount: cart_item_params[:amount])
      else
        cart_items.create!(cart_item_params)
      end
    end
given Hash or MongoidCart::ActsAsProduct instance will be added to current_cart
 @return