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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_without_invoice(self):
 """ Returns transactions that don't have an invoice. We filter out transactions that have children, because those transactions never have invoices - their children are the ones that would each have one invoice. """

        qs = Transaction.objects.filter(
            children__isnull=True, invoice__isnull=True)
        return qs