''' Sets the quantities on each of the products on each of the

        products specified. Raises an exception (ValidationError) if a limit

        is violated. `product_quantities` is an iterable of (product, quantity)

        pairs. '''

        items_in_cart = rego.ProductItem.objects.filter(cart=self.cart)

        product_quantities = list(product_quantities)

        # n.b need to add have the existing items first so that the new

        # items override the old ones.

        all_product_quantities = dict(itertools.chain(

            ((i.product, i.quantity) for i in items_in_cart.all()),

        for voucher in to_remove:

            self.cart.vouchers.remove(voucher)

        # Fix products and discounts

        items = rego.ProductItem.objects.filter(cart=self.cart)

        products = set(i.product for i in items)

        available = set(ProductController.available_products(

            self.cart.user,

            products=products,

        ))

        ''' Calculates all of the discounts available for this product.

        '''

        # Delete the existing entries.

        rego.DiscountItem.objects.filter(cart=self.cart).delete()

        products = [i.product for i in product_items]

        discounts = discount.available_discounts(self.cart.user, [], products)

        # The highest-value discounts will apply to the highest-value

        # products first.

        product_items = self.cart.productitem_set.all()

        product_items = product_items.order_by('product__price')

        product_items = reversed(product_items)

        for item in product_items:

            self._add_discount(item.product, item.quantity, discounts)

    def _add_discount(self, product, quantity, discounts):