private Publisher<WriteResult> insert(StandardOperations.Insert insert) {
    final MongoCollection<Object> collection = (MongoCollection<Object>) this.collection;
    final List<Object> values = insert.values();
    return Flowable.fromPublisher(collection.insertMany(values)).map(r -> WriteResult.empty().withInsertedCount(r.getInsertedIds().size()));
  }
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