根据排序顺序获取文档在集合中的位置 [英] Get document's placement in collection based on sort order

问题描述

我是MongoDB(+猫鼬)的新手.我收集了一些如下所示的高分文件:

I'm new to MongoDB (+Mongoose). I have a collection of highscores with documents that looks like this:

{id: 123, user: 'User14', score: 101}
{id: 231, user: 'User10', score: 400}
{id: 412, user: 'User90', score: 244}
{id: 111, user: 'User12', score: 310}
{id: 221, user: 'User88', score: 900}
{id: 521, user: 'User13', score: 103}

+ thousands more...

现在我正在获得前5名球员，如下所示:

now I'm getting the top 5 players like so:

highscores
    .find()
    .sort({'score': -1})
    .limit(5)
    .exec(function(err, users) { ...code... });

这很棒，但是我也想进行一个查询，例如"user12在高分列表上有什么位置?"

which is great, but I would also like to make a query like "What placement does user12 have on the highscore list?"

以某种方式可以实现查询吗?

Is that possible to achieve with a query somehow?

推荐答案

可以使用 mapReduce ，但是它确实要求您在已排序字段上具有索引，因此，如果您尚未这样做，则首先:

It is possible to do this with mapReduce, but it does require that you have an index on the sorted field, so first, if you already have not done:

db.highscores.ensureIndex({ "score": -1 })

然后您可以执行以下操作:

Then you can do this:

db.highscores.mapReduce(
    function() {
        emit( null, this.user );
    },
    function(key,values) {
        return values.indexOf("User12") + 1;
    },
    {
        "sort": { "score": -1 },
        "out": { "inline": 1 }
    }
)

或者将其更改为您需要返回的信息，而不仅仅是排名"位置.但是，由于基本上是将所有内容放入一个已经按分数排序的大型数组中，所以对于任何合理大小的数据来说，它可能都不是最佳性能.

Or vary that to the information you need to return other than simply the "ranking" position. But since that is basically putting everything into a large array that has already been sorted by score then it probably will not be the best performance for any reasonable size of data.

一个更好的解决方案是维护一个单独的排名"集合，即使它没有进行任何减少，您也可以再次使用mapReduce进行定期更新:

A better solution would be to maintain a separate "rankings" collection, which you can again update periodically with mapReduce, even though it does not do any reducing:

db.highscores.mapReduce(
    function() {
        ranking++;
        emit( ranking, this );
    },
    function() {},
    {
        "sort": { "score": -1 },
        "scope": { "ranking": 0 },
        "out": {
            "replace": "rankings"
        }
    }
)

然后，您可以查询此集合以获取结果:

Then you can query this collection in order to get your results:

db.rankings.find({ "value.user": "User12 })

因此，该排名将在排名"集合的_id字段中包含为已散发"的排名.

So that would contain the ranking as "emitted" in the _id field of the "rankings" collection.

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