def METHOD_NAME(self, attrs):
    if not attrs.get("event"):
        if "currency" not in attrs:
            currencies = list(
                self.organizer.events.order_by('currency').values_list('currency', flat=True).distinct()
            )
            if len(currencies) != 1:
                raise ValidationError({"currency": ["Currency is ambiguous, please set explicitly."]})
            else:
                attrs["currency"] = currencies[0]
    return attrs