lichess-main-1(mongod-2.6.5)[PRIMARY] lichess> db.opening_attempt.aggregate({$group:{_id:'$u',nb:{$sum:1}}},{$sort:{nb:-1}},{$limit:10})
{
  "result": [
    {
      "_id": "dmnq",
      "nb": 2466
    },
    {
      "_id": "optokopper",
      "nb": 1394
    },
    {
      "_id": "nebojsamikic",
      "nb": 1276
    },
    {
      "_id": "still_water",
      "nb": 1011
    },
    {
      "_id": "checksmix",
      "nb": 927
    },
    {
      "_id": "monopo",
      "nb": 923
    },
    {
      "_id": "nimzozo",
      "nb": 845
    },
    {
      "_id": "atani",
      "nb": 807
    },
    {
      "_id": "dolg",
      "nb": 793
    },
    {
      "_id": "kapsarov",
      "nb": 778
    }
  ],
  "ok": 1
}