def set_quantities(self, product_quantities):
        ''' 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 = commerce.ProductItem.objects.filter(cart=self.cart)
        items_in_cart = items_in_cart.select_related(
            "product",
            "product__category",
        )

        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()),
            product_quantities,
        )).items()

        # Validate that the limits we're adding are OK
        products = set(product for product, q in product_quantities)
        try:
            self._test_limits(all_product_quantities)
        except CartValidationError as ve:
            # Only raise errors for products that we're explicitly
            # Manipulating here.
            for ve_field in ve.error_list:
                product, message = ve_field.message
                if product in products:
                    raise ve

        new_items = []
        products = []
        for product, quantity in product_quantities:
            products.append(product)

            if quantity == 0:
                continue

            item = commerce.ProductItem(
                cart=self.cart,
                product=product,
                quantity=quantity,
            )
            new_items.append(item)

        to_delete = (
            Q(quantity=0) |
            Q(product__in=products)
        )

        items_in_cart.filter(to_delete).delete()
        commerce.ProductItem.objects.bulk_create(new_items)
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.