def update_fields(self, record_id, fields):
        """Safely update a number of fields. 'fields' being a
        dictionary with path_tuple: value for only the fields we want
        to change the value of, where path_tuple is a tuple of
        fieldnames indicating the path to the possibly nested field
        we're interested in.
        """
        while True:
            record = self.db[record_id]
            modified = False
            for path, value in fields.items():
                if not isinstance(path, tuple):
                    path = (path,)
                parent = record
                for field in path[:-1]:
                    parent = parent.setdefault(field, {})
                if parent.get(path[-1]) == value:
                    continue
                modified = True
                parent[path[-1]] = value
            if modified:
                try:
                    self.db[record.id] = record
                except ResourceConflict:
                    logger.debug(
                        "Update of record with id %s in database %s failed." % (
                            record_id, self.db.name))
                    continue
            break