Chapter 2:Create/Update Comments

All my unit test pass but I’m still not getting the

code for:
Create/Update Comments: Unable to update comment
Delete Comments: Deletion was performed but unsuccessful

List<Bson> pipeline = new ArrayList<>();
// match stage to find movie
Bson match = Aggregates.match(Filters.eq("_id", new ObjectId(movieId)));
pipeline.add(match);
pipeline.addAll(
                Arrays.asList(new Document("$match",
                                new Document("_id",
                                        new ObjectId(movieId))),
                        new Document("$lookup",
                                new Document("from", "comments")
                                        .append("let",
                                                new Document("id", "$_id"))
                                        .append("pipeline", Arrays.asList(new Document("$match",
                                                new Document("$expr",
                                                        new Document("$eq", Arrays.asList("$movie_id", "$$id"))))))
                                        .append("as", "comments")),
                        new Document("$sort", new Document("comments.date", -1))// this does nothing
                )
);

I guess my problem is sorting the movie comments but I cannot see how to fix…