private Publisher<?> getByKey(StandardOperations.GetByKey getByKey) {
    Bson filter = Mongos.filterById(getByKey.keys());
    return Flowable.fromPublisher(collection.find(filter));
  }
Uses <a href="https://docs.mongodb.com/manual/reference/method/db.collection.replaceOne/">replaceOne</a> operation
with <a href="https://docs.mongodb.com/manual/reference/method/db.collection.bulkWrite/">bulkWrite</a>. Right now has to convert
object to BsonDocument to extract {@code _id} attribute.
criteria/mongo/src/org/immutables/criteria/mongo/MongoSession.java