@Override
    public KeyMetadata metadata() {
      return new KeyMetadata() {
        @Override
        public boolean isKeyDefined() {
          return false;
        }

        @Override
        public boolean isExpression() {
          return false;
        }

        @Override
        public List<Expression> keys() {
          return Collections.emptyList();
        }
      };
    }
Empty extractor which always throws exceptions
criteria/common/src/org/immutables/criteria/backend/KeyExtractors.java