Java Mongo聚合查询

Question

我试图为每个文档计算和保存数组字段的大小。我的代码：

    BasicDBObject query = new BasicDBObject("$pcount", new  BasicDBObject ("$size","$products"));

    ArrayList al = new ArrayList();
    al.add(query);
    AggregateIterable out = mini.aggregate(al);
    MongoCursor cursor = out.iterator();
    while(cursor.hasNext()){
        System.out.println(cursor.next());
    }

错误：命令失败，错误40324：＆＃39;无法识别的管道阶段名称：＆＃39; $ pcount＆＃39;＆＃39;在服务器上