Movie by ID where joined comments are ordered by date in descending order

MovieDao.java

  /**
   * Gets a movie object from the database.
   *
   * @param movieId - Movie identifier string.
   * @return Document object or null.
   */
  public Document getMovie(String movieId) {
    if (!validIdValue(movieId)) {
      return null;
    }
    // TODO> Ticket: Get Comments - implement the lookup stage that allows the comments to
    // retrieved with Movies where comments are ordered by date descending.
    List<Document> pipeline = asList(new Document("$match", new Document("_id", new ObjectId(movieId))),
      new Document("$lookup", new Document("from", "comments")
        .append("let", new Document("movie_id", "$_id"))
        .append("pipeline", asList(new Document("$match",
            new Document("$expr", new Document("$and", singletonList(new Document("$eq", asList("$movie_id", "$$movie_id")))))),
          new Document("$sort", new Document("date", -1)))).append("as", "comments")));

    return moviesCollection.aggregate(pipeline).first();
  }