Using references inside Subquery

[Dufran]

Hello.
I'm trying to add last_activity timestamp on ORM level using subquery, but references only supports passing an object, that seems impossible to do.
Is there any way it's possible to use references inside subquery?(maybe with providing model class and PK?) ?

# * Can't be used
po_activities_subquery = (
            pghistory.models.Events.objects.across(
                po_event,
                po_line_item_event,
                po_note_event,
                po_document_event,
            )
            .references(
                PO.objects.all().filter(
                    id=OuterRef("pk"),
                ),
            )
            .order_by("-pgh_created_at")
            .values("pgh_created_at")[:1]
        )

# * Not working 
po_activities_subquery = (
            pghistory.models.Events.objects.across(
                po_event,
                po_line_item_event,
                po_note_event,
                po_document_event,
            )
            .references(
           OuterRef("pk"),
            )
            .order_by("-pgh_created_at")
            .values("pgh_created_at")[:1]
        )

po_base = PO.objects.all().annotate(
            document_type=Value(UserDocumentType.PURCHASE_ORDER.name),
            document_number=F("order_number"),
            last_activity=Subquery(
                po_activities_subquery,
            ),
        )