docs: def previous_obj(self):
          """Returns a model obj that is the first occurrence of a previous
          obj relative to this object's appointment.
  
          Override this method if not am EDC subject model / CRF.
          """
          previous_obj = None
          if self.previous_visit:
              try:
                  previous_obj = self.model.objects.get(
                      **{f"{self.model.visit_model_attr()}": self.previous_visit}
                  )
              except ObjectDoesNotExist:
                  pass
          return previous_obj