@Override
        public void write(final Key key, final DataWriter writer) {
            // Write single key part.
            final KeyPart keyPart = key.getKeyParts().getFirst();
            keyPart.write(writer);
        }
Creates a flat group key. <GROUP VAL ARRAY>
stroom-query/stroom-query-common/src/main/java/stroom/query/common/v2/KeyFactoryFactory.java