How to get the value repetition count on array of objects for all entries on mongodb aggregation

Question

I have the data structure like this:

{
    "_id" : ObjectId("5c4404906736bd2608e30b5e"),
    "assets": [ 
        {
            "name" : "xa",
            "id"   : 1
        },
        {
            "name" : "xs",
            "id"   : 2
        }
    ]
},
{
    "_id" : ObjectId("5c4404906736bd2608e30b5f"),
    "assets": [ 
        {
            "name" : "xa",
            "id"   : 3
        }
    ]
},
{
    "_id" : ObjectId("5c4404906736bd2608e30b5g"),
    "assets": [ 
        {
            "name" : "xa",
            "id"   : 4
        },
        {
            "name" : "xd",
            "id"   : 5
        },
        {
            "name" : "xs",
            "id"   : 6
        }
    ]
}

Now I want to implement the MongoDB aggregation by which I got the Answer like this:

[
 {
  "assets": "xa",
  "count": 3
 },
 {
  "assets": "xs",
  "count": 2
 },
 {
  "assets": "xd",
  "count": 1
 },
]

I have to get this done by javascript but need to implement this on aggregation. My code for acheiveing with js is like this for set of array of object ie

var arr = [
  { asset: "xa" },
  { asset: "xs" },
  { asset: "xa" },
  { asset: "xs" },
  { asset: "xa" },
  { asset: "xd" }
];

var userDict = arr.reduce((acc, el) => {
  if (!acc.hasOwnProperty(el.asset)) {
    acc[el.asset] = { count: 0 };
  }
  acc[el.asset].count++;
  return acc;
}, {});

var result = Object.entries(userDict).map(([k, v]) => ({
  asset: k,
  count: v.count
}));
console.log(result);

Any help is really appreciated

Answer 1

You can $unwind assets before applying $group with count:

db.col.aggregate([
    {
        $unwind: "$assets"
    },
    {
        $group: {
            _id: "$assets.name",
            count: { $sum: 1 }
        }
    },
    {
        $project: {
            _id: 0,
            asset: "$_id",
            count: 1
        }
    }
])