product_items = product_items.order_by(

            "product__category__order", "product__order"

        )

        discount_items = rego.DiscountItem.objects.filter(cart=cart)

        invoice_value = Decimal()

        for item in product_items:

            product = item.product

            line_item = rego.LineItem.objects.create(

                invoice=invoice,

                description="%s - %s" % (product.category.name, product.name),

                quantity=item.quantity,

                price=product.price,

                product=product,

            )

            invoice_value += line_item.quantity * line_item.price

        for item in discount_items:

            line_item = rego.LineItem.objects.create(

                invoice=invoice,

                description=item.discount.description,

                quantity=item.quantity,

                price=cls.resolve_discount_value(item) * -1,

                product=item.product,

            )

            invoice_value += line_item.quantity * line_item.price

        invoice.value = invoice_value

        invoice.save()

        return invoice

    def can_view(self, user=None, access_code=None):

        ''' Returns true if the accessing user is allowed to view this invoice,

        or if the given access code matches this invoice's user's access code.

        '''

        if user == self.invoice.user:

            return True

        if user.is_staff:

            return True

        if self.invoice.user.attendee.access_code == access_code:

            return True

        return False

    def _refresh(self):

        ''' Refreshes the underlying invoice and cart objects. '''

        self.invoice.refresh_from_db()

        if self.invoice.cart:

            self.invoice.cart.refresh_from_db()

    def validate_allowed_to_pay(self):

        ''' Passes cleanly if we're allowed to pay, otherwise raise

        a ValidationError. '''

        self._refresh()

        if not self.invoice.is_unpaid:

            raise ValidationError("You can only pay for unpaid invoices.")

        if not self.invoice.cart:

            return

        if not self._invoice_matches_cart():

            raise ValidationError("The registration has been amended since "

                                  "generating this invoice.")

        CartController(self.invoice.cart).validate_cart()

    def total_payments(self):

        ''' Returns the total amount paid towards this invoice. '''

        payments = rego.PaymentBase.objects.filter(invoice=self.invoice)

        total_paid = payments.aggregate(Sum("amount"))["amount__sum"] or 0

        return total_paid

    def update_status(self):

        ''' Updates the status of this invoice based upon the total

        payments.'''

        old_status = self.invoice.status

        total_paid = self.total_payments()

        num_payments = rego.PaymentBase.objects.filter(

            invoice=self.invoice,

        ).count()

        remainder = self.invoice.value - total_paid

        if old_status == rego.Invoice.STATUS_UNPAID:

            # Invoice had an amount owing

            if remainder <= 0:

                # Invoice no longer has amount owing

                self._mark_paid()

            elif total_paid == 0 and num_payments > 0: