@Override
    protected void onSerialize(Serializer buf) {
        super.onSerialize(buf);
        putUtf8(buf, fieldName);
        serializeOptional(buf, result);
    }
Sets the name of the field whose value to retrieve.

@param fieldName The field name to set.
searchlib/src/main/java/com/yahoo/searchlib/expression/DocumentFieldNode.java