如何在Mongo中使用Java聚合三个集合？

Question

如何在Mongo中使用Java聚合三个集合？

这是我的代码（或者最好是没有dataTemp的代码），

但是它返回空结果，如何解决？

    Morphia morphia = new Morphia();
    morphia.mapPackage("com.x.core.order.report");
    MongoClient bean = new AnnotationConfigApplicationContext(SpringMongoConfig.class).getBean(MongoClient.class);
    Datastore datastore = morphia.createDatastore(bean, "x");
    datastore.ensureIndexes();

    User user = new User();
    user.setEmail("eeeeeeee");
    Order order = new Order();
    order.setComment("cccccccc");
    datastore.save(user);
    order.setProducerAssigneeId(user.getId());
    datastore.save(order);
    MongoDatabase db = bean.getDatabase("x");
    MongoCollection<Document> users = db.getCollection("orders");
    BasicDBObject match = new BasicDBObject("$match",
            new BasicDBObject("_id", order.getId())
    );
    BasicDBObject lookupFields = new BasicDBObject("from", "users");
    lookupFields.put("localField", "producerAssigneeId");
    lookupFields.put("foreignField", "_id");
    lookupFields.put("as", "producerAssignee");
    BasicDBObject lookup = new BasicDBObject("$lookup", lookupFields);

    BasicDBObject out = new BasicDBObject("$out", "datasetTemp");
    List<BasicDBObject> pipeline = Arrays.asList(match,  lookup, out);
    AggregateIterable<Document> resultAgg = users.aggregate(pipeline);
    MongoCollection<Document>  tmp = db.getCollection("datasetTemp");
    tmp.find().first();//...