count total number sub document in Array

Question

I want count of sub document in array, without using $unwind and $group , if possible.

[{
    "_id" : ObjectId("5aefead0227bc943df3fedff"),
    "doc" : [ 
        {
            "_id" : ObjectId("5af0e877227bc943df401337"),
            "shared_with" : [ 
                {
                    "user_id" : ObjectId("5ad5e57b473e2606e0d443c3"),
                    "_id" : ObjectId("5af0e877227bc943df401339")
                }, 
                {
                    "user_id" : ObjectId("5adbf029b2da8e380b9321b6"),
                    "_id" : ObjectId("5af0e877227bc943df401338")
                }
            ]
        }, 
        {
            "_id" : ObjectId("5b4d856702d7f52974aab962"),
            "shared_with" : [ 
                {
                    "user_id" : ObjectId("5ac7083ce56c9d304f2fa533"),
                    "_id" : ObjectId("5b4d856702d7f52974aab963")
                }
            ]
        }
    ]
}]

above is the result i'm getting after lookup, i want to count total number of sub document ('shared_with') inside array ('doc') with documents,

so my desired output is as follow,

[{
   "_id" : ObjectId("5aefead0227bc943df3fedff"),
   "count":3`enter code here`
}]

Ashh · Accepted Answer · 2018-07-17 06:52:18Z

2

You can try $map aggregation first to loop over the doc and find the $size of the shared_with array and then count all the shared_with with $sum aggregation

db.collection.aggregate([
  { "$project": {
    "doc": {
      "$map": {
        "input": "$doc",
        "as": "do",
        "in": {
          "_id": "$$do._id",
          "count": { "$size": "$$do.shared_with" }
        }
      }
    }
  }},
  { "$project": {
    "count": { "$sum": "$doc.count" }
  }}
])

Output

[
  {
    "_id": ObjectId("5aefead0227bc943df3fedff"),
    "count": 3
  }
]

answered Jul 17, 2018 at 6:52

Ashh

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s7vr · Accepted Answer · 2018-07-17 12:19:49Z

1

You can use $reduce to count the inner subdocuments.

Something like

db.colname.aggregate([
  {"$project":{
    "count":{
      "$reduce":{
        "input":"$doc",
        "initialValue":0,
        "in":{"$add":["$$value",{"$size":"$$this.shared_with"}]}
      }
    }
  }}
])

answered Jul 17, 2018 at 12:19

s7vr

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