gistfile1.txt

ns29899(mongod-2.4.8) lichess> db.puzzle_attempt.aggregate({$group:{_id:'$u', nb:{$sum:1}}},{$sort:{nb:-1}},{$limit:10})
{
  "result": [
    {
      "_id": "u235",
      "nb": 813
    },
    {
      "_id": "aakarsh01",
      "nb": 449
    },
    {
      "_id": "trialb",
      "nb": 434
    },
    {
      "_id": "meikeschlecker",
      "nb": 426
    },
    {
      "_id": "thiago",
      "nb": 395
    },
    {
      "_id": "rzenaikrzys",
      "nb": 382
    },
    {
      "_id": "emperator",
      "nb": 356
    },
    {
      "_id": "majid-ganji",
      "nb": 327
    },
    {
      "_id": "lionne2222",
      "nb": 306
    },
    {
      "_id": "hasanss",
      "nb": 294
    }
  ],
  "ok": 1
}