public static boolean isFieldValues(FieldPathUpdate update) {
        if (!(update instanceof AssignFieldPathUpdate)) return false;
        // Only consider field path updates that touch a top-level field as 'complete',
        // as these may be converted to regular field value updates.
        return ((update.getFieldPath().size() == 1)
                && update.getFieldPath().get(0).getType() == FieldPathEntry.Type.STRUCT_FIELD);
    }
Returns true if this update can be represented as a set of document field values.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/FieldPathUpdateHelper.java