def update(self):
        """Update the position of the mark in the collection.

        :return: this object, for chaining
        :rtype: Mark
        """
        rec = self._c.find_one({}, {self._fld: 1}, sort=[(self._fld, -1)], limit=1)
        if rec is None:
            self._pos = self._empty_pos()
        elif not self._fld in rec:
            _log.error("Tracking field not found. field={} collection={}"
                       .format(self._fld, self._c.name))
            _log.warn("Continuing without tracking")
            self._pos = self._empty_pos()
        else:
            self._pos = {self._fld: rec[self._fld]}
        return self
Update the position of the mark in the collection.

        :return: this object, for chaining
        :rtype: Mark