@Override
    protected void onDeserialize(Deserializer buf) {
        super.onDeserialize(buf);
        fieldName = getUtf8(buf);
        result = (ResultNode)deserializeOptional(buf);
    }
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