))

        not_available = products - available

        zeros = [(product, 0) for product in not_available]

        self.set_quantities(zeros)

    @transaction.atomic

    def recalculate_discounts(self):

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

        '''

        # Delete the existing entries.

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

        product_items = self.cart.productitem_set.all().select_related(

        )

        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 = reversed(product_items)

        for item in product_items:

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

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

        ''' Applies the best discounts on the given product, from the given

        discounts.'''

        def matches(discount):

            ''' Returns True if and only if the given discount apples to

            our product. '''

            if isinstance(discount.clause, conditions.DiscountForCategory):

                return discount.clause.category == product.category

            else:

                return discount.clause.product == product