public FieldValue getFieldValue(StructuredFieldValue doc) {
        Object raw = extract.get(doc);
        return raw == null ? null : getDataType().createFieldValue(raw);
    }
This adds an Extractor to the Field that can be used to access the backed value
used in the concrete document types.
@author baldersheim
document/src/main/java/com/yahoo/document/ExtendedField.java