Java MongoDB聚合不能使用$limit两次

我有以下

MongoDB

collection“

Games

”：

我想将以下MySQL查询映射到MongoDB

String query = "SELECT idplayer, COUNT(idplayer) AS countplayer "
                    + "FROM (SELECT b.idgame, b.idplayer "
                    + "FROM associations a, associations b "
                    + "WHERE a.idplayer=? "
                    + "AND b.idgame=a.idgame "
                    + "AND b.idplayer <> ? "
                    + "ORDER BY b.idgame DESC LIMIT 1000) as c"
                    + " GROUP BY idplayer "
                    + "ORDER BY countplayer DESC LIMIT 5;";

---

请帮我解决这个问题。谢谢

第一次

组

操作后，字段

游戏

不在结果中，因此基于字段

游戏

的第二次

组

操作不起作用

为了获得更高效的查询，您应该对聚合操作重新排序，以便尽早减少数据。在解开玩家名单之前，我移动了游戏的匹配，因此不需要第二组

在mongo shell中，聚合操作如下所示：

// playerId to search for coplayers
var playerId = "DoomY9999"

db.game.aggregate([
    // First $match can take advantage of suitable index if available

    // Find all games that playerid X has played
    { $match : { "playersList.playerid" : playerId } },

    // Sort by most recent games (gameid descending)
    { $sort : { "_id.gameid" : -1 } },

    // Limit number of games to examine
    { $limit : 1000 },

    // Create a stream of documents from the playersList array
    { $unwind : "$playersList" },

    // Match players except for playerid X
    { $match : { "playersList.playerid" : {$ne : playerId }} },

    // Count number of games each player has played
    { $group : {
        _id : "$playersList.playerid",
        count : { $sum : 1 }
    }},

    // Sort by most frequent players (count descending)
    { $sort : { "count" : -1 } },

    // Limit results to 5 players
    { $limit : 5 },

    // Rename the result fields
    { $project : {
        _id : 0,
        coplayer : "$_id",
        count : 1
    }}
])

 //build the query
    DBObject match1 = new BasicDBObject("$match", new BasicDBObject("playersList.playerid",_playerid));
    DBObject unwind = new BasicDBObject("$unwind", "$playersList");
    DBObject match2 = new BasicDBObject("$match", new BasicDBObject("playersList.playerid",new BasicDBObject("$ne",_playerid)));

    DBObject groupFields = new BasicDBObject("_id","$playersList.playerid");
    groupFields.put("times", new BasicDBObject("$sum",1));
    DBObject group = new BasicDBObject("$group", groupFields);
    DBObject sort = new BasicDBObject("$sort", new BasicDBObject("times",-1) );
    DBObject limit = new BasicDBObject("$limit", 5 );

    DBObject group2 = new BasicDBObject("$group", "gameid");
    DBObject sort2 = new BasicDBObject("$sort", new BasicDBObject("gameid",-1) );
    DBObject limit2 = new BasicDBObject("$limit", 1000 );

    DB db = mongoDb;
    DBCollection coll = db.getCollection("games");

    //aggregation query
    //THIS WORKS
    AggregationOutput output = coll.aggregate( match1, unwind, match2, group, sort, limit);

    //THIS DOESN'T WORK!
     AggregationOutput output = coll.aggregate( match1, unwind, match2, group, sort, limit, group2, sort2, limit2); 

// playerId to search for coplayers
var playerId = "DoomY9999"

db.game.aggregate([
    // First $match can take advantage of suitable index if available

    // Find all games that playerid X has played
    { $match : { "playersList.playerid" : playerId } },

    // Sort by most recent games (gameid descending)
    { $sort : { "_id.gameid" : -1 } },

    // Limit number of games to examine
    { $limit : 1000 },

    // Create a stream of documents from the playersList array
    { $unwind : "$playersList" },

    // Match players except for playerid X
    { $match : { "playersList.playerid" : {$ne : playerId }} },

    // Count number of games each player has played
    { $group : {
        _id : "$playersList.playerid",
        count : { $sum : 1 }
    }},

    // Sort by most frequent players (count descending)
    { $sort : { "count" : -1 } },

    // Limit results to 5 players
    { $limit : 5 },

    // Rename the result fields
    { $project : {
        _id : 0,
        coplayer : "$_id",
        count : 1
    }}
])