@Override
    public void onDeserialize(Deserializer data) {
        if (data instanceof FieldReader) {
            deserialize(null, (FieldReader) data);
        } else {
            throw new IllegalArgumentException("I am not able to deserialize from " + data.getClass().getName());
        }
    }
Using the given field path, digs through the document and returns the matching field value.
If the field path resolves to multiple values, returns an ArrayFieldValue containing the
values.
document/src/main/java/com/yahoo/document/datatypes/FieldValue.java