mongodb java aggregate return empty result

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i need to use mongodb java aggregate function to detect duplicated documents in collection, this is the query i ran from command line and it worked:

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

and this is the java code i wrote:

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));

but it always return empty result. can anyone tell me what's wrong?

asked 21 secs ago

user468587

702