def get_context_data(self, **kwargs):
    context = super(InvoiceDetail, self).dispatch(*args, **kwargs)
    request = self.request

    if request.GET:
        try:
            invoice_selector = request.GET.get('invoice_selector', None)
            invoice = Invoice.objects.get(id = int(invoice_selector) ) # either a string representing a number or 'add invoice'
            context_object_model = invoice
        except ValueError:
            return HttpResponseRedirect(reverse('accounting:add_invoice'))

    return context