Returns transaction totals grouped by Payee.

def get_totals_by_payee(self, account, start_date=None, end_date=None):
        """
        Returns transaction totals grouped by Payee.

        """
        qs = Transaction.objects.filter(account=account, parent__isnull=True)
        qs = qs.values('payee').annotate(models.Sum('value_gross'))
        qs = qs.order_by('payee__name')
        return qs