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()