def _items(self, cart_status, category=None):

        ''' Aggregates the items that this user has purchased.

        Arguments:

            cart_status (int or Iterable(int)): etc

            category (Optional[models.inventory.Category]): the category

                of items to restrict to.

        Returns:

            [ProductAndQuantity, ...]: A list of product-quantity pairs,

                aggregating like products from across multiple invoices.

        '''

        if not isinstance(cart_status, Iterable):

            cart_status = [cart_status]

        status_query = (

            Q(productitem__cart__status=status) for status in cart_status

        )

        in_cart = Q(productitem__cart__user=self.user)

        in_cart = in_cart & reduce(operator.__or__, status_query)

        quantities_in_cart = When(

            in_cart,

            then="productitem__quantity",

        )

        quantities_or_zero = Case(

            quantities_in_cart,

            default=Value(0),

        )

        products = inventory.Product.objects

        if category:

            products = products.filter(category=category)

        products = products.select_related("category")

        products = products.annotate(quantity=Sum(quantities_or_zero))

        products = products.filter(quantity__gt=0)

        out = []

        for prod in products:

            out.append(ProductAndQuantity(prod, prod.quantity))

        return out