static KeyExtractor.Factory generic(UnaryOperator<Object> extractor) {
    Objects.requireNonNull(extractor, "extractor");
    return entityType -> new KeyExtractors.Generic(entityType, extractor);
  }
Creates extractor from a generic java function. Metadata on keys will not be available.
criteria/common/src/org/immutables/criteria/backend/KeyExtractor.java