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": 1107
    },
    {
      "_id": "rzenaikrzys",
      "nb": 1077
    },
    {
      "_id": "eqsx",
      "nb": 1023
    },
    {
      "_id": "thiago",
      "nb": 954
    },
    {
      "_id": "trialb",
      "nb": 935
    },
    {
      "_id": "smok",
      "nb": 780
    },
    {
      "_id": "lycanthrope",
      "nb": 718
    },
    {
      "_id": "mots",
      "nb": 706
    },
    {
      "_id": "srxa",
      "nb": 616
    },
    {
      "_id": "vexch",
      "nb": 593
    }
  ],
  "ok": 1
}