$group with nested array element

Question

would like to know how to use MongoDB's aggregation to collectively summarize a count of each reason_id:

I would like to get known that there are 2 counts for having "reason_id = KW7Kcsv7835YZeE3n", and 1 count for having "reason_id = KNcKQCjhFzha3oLfE".

Here is my data:

[
  {
    "_id": "2DLQFJLbZScBXpSam",
    "toilet_id": "bJsyfh3TCvpTzE2mJ",
    "reason_ids": [
      "KNcKQCjhFzha3oLfE",
      "KW7Kcsv7835YZeE3n"
    ],
    "score_id": null,
    "toilet": {
      "_id": "bJsyfh3TCvpTzE2mJ",
      "name": "Toilet_M_1",
      "gender": "m",
      "mac_address": "11:11:11:11:11:11"
    }
  },

  {
    "_id": "akjsbcjascklsacas",
    "toilet_id": "bJsyfh3TCvpTzE2mJ",
    "reason_ids": [
      "KW7Kcsv7835YZeE3n"
    ],
    "score_id": null,
    "toilet": {
      "_id": "fsgndgklndsdsdsd",
      "name": "Toilet_F_1",
      "gender": "f",
      "mac_address": "11:11:11:11:11:11"
    }
  },
]

You can use db.collectionname.aggregate([ {"$unwind":"$reason_ids"}, {"$group":{"_id":"$reason_ids","count":{"$sum":1}}} ]) — s7vr
– s7vr, Commented Jul 12, 2018 at 2:38

Ashh · Accepted Answer · 2018-07-12 03:22:00Z

1

You can try this

db.collection.aggregate([
  {
    "$unwind": "$reason_ids"
  },
  {
    "$group": {
      "_id": "$reason_ids",
      "count": {
        "$sum": 1
      }
    }
  }
])

Output

[
  {
    "_id": "KW7Kcsv7835YZeE3n",
    "count": 2
  },
  {
    "_id": "KNcKQCjhFzha3oLfE",
    "count": 1
  }
]

Try it here

answered Jul 12, 2018 at 3:22

Ashh

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