def get_extra_couchdbs(config, couch_database_url):
    """
    Create a mapping from database prefix to database url

    :param config:              list of database strings or tuples
    :param couch_database_url:  main database url
    """
    extra_dbs = {}
    for row in config:
        if isinstance(row, tuple):
            _, postfix = row
            extra_dbs[postfix] = '%s__%s' % (couch_database_url, postfix)

    return extra_dbs