private Publisher<WriteResult> deleteByKey(StandardOperations.DeleteByKey deleteByKey) {
    if (deleteByKey.keys().isEmpty()) {
      return Flowable.just(WriteResult.empty());
    }

    Bson filter = Mongos.filterById(deleteByKey.keys());
    return Flowable.fromPublisher(collection.deleteMany(filter))
            .map(r -> WriteResult.empty().withDeletedCount(r.getDeletedCount()));
  }
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