cart_products_controller.rb

      # If the cart already has this product in it,
    if existing_cart_product = cart.cart_products.find_by_part_number_id(@cart_product.part_number_id)
        # And if the current quantity in cart exceeds or equals the available quantity
      if existing_cart_product.quantity >= available_quantity
        # Set the quantity in cart to the available quantity, and add 0 of the requested quantity to the cart.
        existing_cart_product.update_attributes(:quantity => available_quantity)
        @addl_msg << "Quantity has been depleted for this item. There are #{existing_cart_product.quantity} units in your cart. "
        @cart_product.quantity = 0
      end
    end