@Override
    public String toString() {
        if (resolved != null) return resolved.toString();
        return modelId.orElse("\"\"") + " " +
               url.map(UrlReference::value).orElse("\"\"") + " " +
               path.map(FileReference::value).orElse("\"\"");
    }
Returns this on the format accepted by valueOf
config-lib/src/main/java/com/yahoo/config/ModelReference.java