@Override
    public DocumentUpdate getOutput() {
        Document doc = fwdAdapter.getFullOutput();
        DocumentUpdate upd = new DocumentUpdate(doc.getDataType(), doc.getId());
        upd.addFieldPathUpdate(update);
        return upd;
    }
No-op update adapter which simply passes through the input update unchanged.
I.e. getOutput() will return a DocumentUpdate containing only the FieldPathUpdate
the IdentityFieldPathUpdateAdapter was created with. All other applicable calls are
forwarded to the provided DocumentAdapter instance.

This removes the need for a potentially lossy round-trip of update -&gt; synthetic document -&gt; update.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/IdentityFieldPathUpdateFieldValues.java