@Override
    public FieldValues setOutputValue(String fieldName, FieldValue fieldValue, Expression exp) {
        return fwdAdapter.setOutputValue(fieldName, fieldValue, exp);
    }
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