MongoDB: aggregate grouping objects in array inside of an array of objects

Question

I need to group the objects inside of votes by mallId inside of a mongodb aggreate pipeline. Been searching for an answer for ages, but I am not too familiar with mongodb so I am kind of lost. Can anyone help me out?

     [
      {
        "_id": "Specialty Retailer",
        "votes": [
          {
            "mallId": "59ddea3718a03a4e81f289f5",
            "count": 13
          },
          {
            "mallId": "59ddea3718a03a4e81f289f5",
            "count": 270
          },
          {
            "mallId": "59ddea3718a03a4e81f289f5",
            "count": 13
          },
          {
            "mallId": "59ddea3718a03a4e81f289f5",
            "count": 2
          },
          {
            "mallId": "59ddea3718a03a4e81f289f5",
            "count": 15
          },
          {
            "mallId": "59ddea3718a03a4e81f289f5",
            "count": 2
          }
     ]
   }
]

Answer 1

Given the following document structure

{
    "_id" : ObjectId("5a020ea06216d24dd6050a07"),
    "arr" : [ 
        {
            "_id" : "Specialty Retailer",
            "votes" : [ 
                {
                    "mallId" : "59ddea3718a03a4e81f289f5",
                    "count" : 13
                }, 
                {
                    "mallId" : "59ddea3718a03a4e81f289f5",
                    "count" : 270
                }
            ]
        }
    ]
}

this query should get you going:

db.collection.aggregate({
    $unwind: "$arr"
}, {
    $unwind: "$arr.votes"
}, {
    $group: {
        "_id": {
            "_id": "$_id",
            "arrId": "$arr._id",
            "mallId": "$arr.votes.mallId"
        },
        "totalCount": {
            $sum: "$arr.votes.count"
        }
    }
})