private Publisher<?> getByKey(StandardOperations.GetByKey op) {
      List<Object> result = new ArrayList<>();
      for (Object key: op.keys()) {
        Object value = store.get(key);
        if (value != null) {
          result.add(value);
        }
      }

      return Flowable.fromIterable(result);
    }
mapping between class and its store
criteria/inmemory/src/org/immutables/criteria/inmemory/InMemoryBackend.java