How to get value from objects in array of objects?

Question

I'm making an aggregation and I'm at a step where I have the following:

˅ typeCount: Array
    ˅ 0: Object
        _id: "System"
        count: 1002
    ˅ 1: Object
        _id: "Application"
        count: 3065
˅ affectedComputers: Array
    ˅ 0: Object
        _id: ObjectId('...')
        count: 1
    ˅ 1: Object
        _id: ObjectId('...')
        count: 1
    ...

Now I'm trying to get the following result:

application: 3065
system: 1002
affectedComputers: 2951

The affectedComputers array is a group by computers, so I just use $size to get the count of computers. But I don't know how to get the Application and System count... And keep in mind that they can also be 0 if there is no instances when I group.

I've done the following:

{ 
    $project {
        'affectedComputers': { $size: '$affectedComputers'}
    }
}

This only gives me the affected computers count, how do I get the System and Application count if there are any, if not then get it as 0?

Answer 1

Easiest way to do this generically is just to use $arrayToObject and convert the whole array, You can also define it manually field by field, here's how to do the first approach:

db.collection.aggregate([
  {
    $replaceRoot: {
      newRoot: {
        $mergeObjects: [
          {
            "affectedComputers": {
              $size: "$affectedComputers"
            }
          },
          {
            "$arrayToObject": {
              $map: {
                input: "$typeCount",
                in: {
                  k: "$$this._id",
                  v: "$$this.count"
                }
              }
            }
          }
        ]
      }
    }
  }
])

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