return set(i.category for i in available)

    def user_quantity_remaining(self, user):

        ''' Returns the number of items from this category that the user may

        add in the current cart. '''

        cat_limit = self.category.limit_per_user

        if cat_limit is None:

            # We don't need to waste the following queries

            return 99999999

        carts = rego.Cart.objects.filter(

            user=user,

            active=False,

            released=False,

        )

        items = rego.ProductItem.objects.filter(

            cart__in=carts,

            product__category=self.category,

        )

        cat_count = items.aggregate(Sum("quantity"))["quantity__sum"] or 0