def _update_value_inert(self, index, value):
        """
        is only called by _update_inert
        """
        # get field descriptor
        field_descriptor = self._table._dev_descriptor.get_field_descriptor(index)

        # prepare value
        value = field_descriptor.deserialize(value, index)

        # unregister previous link if relevant
        if isinstance(value, Link):
            # de-activate current link if any
            current_link = self._data.get(index)
            if current_link is not None:
                current_link.unregister()

        # unregister previous hook if relevant
        if isinstance(value, RecordHook):
            current_record_hook = self._data.get(index)
            if current_record_hook is not None:
                current_record_hook.unregister()

        # unregister previous external file if relevant
        if isinstance(value, ExternalFile):
            current_external_file = self._data.get(index)
            if current_external_file is not None:
                current_external_file._dev_unregister()

        # if None remove and leave
        if value in (None, NONE_RECORD_HOOK, NONE_LINK, NONE_EXTERNAL_FILE):
            # we don't check required, because this method is called by _update_inert which does the job
            self._dev_set_none_without_unregistering(index, check_not_required=False)
            return

        # if relevant, store current pk to signal table
        old_hook = None
        if index == 0 and not self._table._dev_auto_pk:
            old_hook = self._data.get(0)  # we use get, because record may not have a pk yet if it is being created

        # set value
        self._data[index] = value

        # signal pk update if relevant
        if old_hook is not None:
            self._table._dev_record_pk_was_updated(old_hook.target_value)
is only called by _update_inert