mongodb java聚合返回空结果

Question

我需要使用mongodb java聚合函数来检测集合中的重复文档，这是我从命令行运行的查询，它可以工作：

db.placements.aggregate(
    {$group:{ "_id: "$campaign", total:{$sum:1}}}, 
    {$match: {total: {$gt:2}}}, 
    {$limit:10}, 
    {$skip:0}
);

这是我写的java代码：

DBObject groupFields = new BasicDBObject("_id", "$campaign");
groupFields.put("total", new BasicDBObject("$sum", 1));     
DBObject group = new BasicDBObject("$group", groupFields);

DBObject matchFields = new BasicDBObject("total", new BasicDBObject("$gt", 2));
DBObject match = new BasicDBObject("$match", matchFields);

collection.aggregate(match, group, new BasicDBObject("$skip", 0), new BasicDBObject("$limit", 10));

但它总是返回空结果。任何人都可以告诉我什么是错的？