models.py

@property
def notes(self):
    order_by = "pk"
    if self.note_order:
        pks = self.note_order.split("|")
        order_by = models.Case(*[
            models.When(pk=pk, then=pos)
            for pos, pk in enumerate(pks)
        ])
    return self.note_set.all().order_by(order_by)