mongodb에서 한 필드 값을 그룹화하고 다른 필드를 정렬하는 방법

요구 사항을 코드로 변환하면 다음 파이프 라인 (mongo 셸 버전)이 제공됩니다.

// 1. Check for documents which have curFlag true
var match = { "$match": { "curFlag": true } };

// 2. GroupBy owner and display the number of files that particular user is handling
var group = {
    "$group": {
        "_id": "$meta.owner",
        "count": { "$sum": 1 }
    }
};

// 3. Display the user who is handling large no. of files
var sort = { "$sort": { "count": -1 } },
    limit = { "$limit": 1 };

// Run pipeline
db.mycollection.aggregate([match, group, sort, limit])

자바 구현 :

public class JavaAggregation {
    public static void main(String args[]) throws UnknownHostException {

        MongoClient mongo = new MongoClient();
        DB db = mongo.getDB("test");

        DBCollection coll = db.getCollection("mycollection");

        // 1. Check for documents which have curFlag true
        DBObject match = new BasicDBObject("$match",
                            new BasicDBObject("curFlag", true)
                        );

        // 2. GroupBy owner and display the number of files per user
        DBObject groupFields = new BasicDBObject( "_id", "$meta.owner");
        groupFields.put("count", new BasicDBObject( "$sum", 1));
        DBObject group = new BasicDBObject("$group", groupFields);

        // 3. Display the user who is handling large no. of files
        DBObject sort = new BasicDBObject("$sort", 
                            new BasicDBObject("count", -1)
                        );
        DbObject limit = new BasicDBObject("$limit", 1);

        List<DBObject> pipeline = Arrays.asList(match, group, sort, limit);

        AggregationOutput output = coll.aggregate(pipeline);

        for (DBObject result : output.results()) {
            System.out.println(result);
        }
    }
}