code: def save(self, mark):
          
          self._check_exists()
          obj = mark.as_dict()
          try:
              # Make a 'filter' to find/update existing record, which uses
              # the field name and operation (but not the position).
              filt = {k: obj[k] for k in (mark.FLD_FLD, mark.FLD_OP)}
              _log.debug("save: upsert-spec={} upsert-obj={}".format(filt, obj))
              self._track.update(filt, obj, upsert=True)
          except pymongo.errors.PyMongoError as err:
              raise DBError("{}".format(err))
docs: Save a position in this collection.
  
          :param mark: The position to save
          :type mark: Mark
          :raises: DBError, NoTrackingCollection