mongoCollection.aggregate(aggregatePipeline)
    .map(document -> {
      Document group = document.get("_id", Document.class);
      return new Document("chat", group).append("count", document.get("count"));
    })
    .into(new ArrayList<>(), (results, e) -> {
      Document document = new Document("chats", results);
      System.out.println(document.toJson());
    });

mongoCollection.aggregate(aggregatePipeline)
    .map[Document](doc => Document("chat" -> doc.get("_id"), "count" -> doc.get("count")))
    .subscribe((doc: Document) => println(doc.toJson))