是的，您可以使用聚合框架来做到这一点.它不会很漂亮，但是它仍然会比使用mapreduce更快.

Yes, you can do this with aggregation framework. It won't be pretty, but then it'll still be much faster than with mapreduce...

简而言之(输出的格式与您提供的格式不同，但内容相同):

Here it is in a nutshell (output a different format than what you give but same content):

> group1 = {
    "$group" : {
        "_id" : "$race",
        "class" : {
            "$push" : "$class"
        },
        "count" : {
            "$sum" : 1
        }
    }
};
> unwind = { "$unwind" : "$class" };
> group2 = {
    "$group" : {
        "_id" : "$class",
        "classCount" : {
            "$sum" : 1
        },
        "races" : {
            "$push" : {
                "race" : "$_id",
                "raceCount" : "$count"
            }
        }
    }
};
> unwind2 = { "$unwind" : "$races" };
> group3 ={
    "$group" : {
        "_id" : 1,
        "classes" : {
            "$addToSet" : {
                "class" : "$_id",
                "classCount" : "$classCount"
            }
        },
        "races" : {
            "$addToSet" : "$races"
        }
    }
};
> db.races.aggregate(group1, unwind, group2, unwind2, group3);
{
    "result" : [
        {
            "_id" : 1,
            "classes" : [
                {
                    "class" : "Fighter",
                    "classCount" : 1
                },
                {
                    "class" : "Hunter",
                    "classCount" : 3
                },
                {
                    "class" : "Rogue",
                    "classCount" : 1
                }
            ],
            "races" : [
                {
                    "race" : "Naga",
                    "raceCount" : 2
                },
                {
                    "race" : "Halfling",
                    "raceCount" : 3
                }
            ]
        }
    ],
    "ok" : 1
}