How to match a array object and nested array object in mongodb without using unwind

Question

My json data will look like this

{
     "data": [
        {
            "_id": "621e139dd65883978081b6a4",
            "videoId": 29,
            "__v": 0,
            "createdAt": "2022-03-01T12:37:49.734Z",
            "updatedAt": "2022-03-04T08:47:21.481Z",
            "videoComments": [
                {
                    "comment": "happy",
                    "userId": 15,
                    "commentTime": "2022-03-01T12:37:49.734Z",
                    "userName": "user1646127068323",
                    "deletedbyowner": "FALSE",
                    "uid": "7TRmiNZ90nfyXupJIzlnBkKFnm03",
                    "_id": "621e139d8e4195079c864818",
                    "replyComments": [
                        {
                            "replyComment": "reply check",
                            "replyCommentTime": "2022-03-01T12:44:53.193Z",
                            "replyDeletedbyowner": "FALSE",
                            "_id": "621e154557fa7045e3425410"
                        }
                    ]
                },
                {
                    "comment": "happy",
                    "userId": 15,
                    "commentTime": "2022-03-01T12:38:35.116Z",
                    "userName": "user1646127068323",
                    "deletedbyowner": "FALSE",
                    "uid": "7TRmiNZ90nfyXupJIzlnBkKFnm03",
                    "_id": "621e13cb7d2f6265aa7ff7ac",
                    "replyComments": []
                },
                {
                    "comment": "happy",
                    "userId": 38,
                    "commentTime": "2022-03-01T12:39:26.064Z",
                    "userName": "user1646127068323",
                    "deletedbyowner": "FALSE",
                    "uid": "7TRmiNZ90nfyXupJIzlnBkKFnm03",
                    "_id": "621e13fe7d2f6265aa7ff7be",
                    "replyComments": [
                        {
                            "replyComment": "reply check2",
                            "replyCommentTime": "2022-03-04T08:47:21.479Z",
                            "replyDeletedbyowner": "FALSE",
                            "_id": "6221d21945208781d16377f9"
                        }
                    ]
                }
            ]
        }
    ]
}

I need to match some conditions that I mentioned below :

match "videoId" == "29"
then match "videoComments.deletedbyowner" == "FALSE"
if I match second condition then I need to match "videoComments.replyComments.replyDeletedbyowner" == "FALSE"
I need to sort the result for the user who is viewing the comment, in my case it is userId:38

So I need to show this particular comment at first.(my comment should be at first when I am viewing the comment list). I can't use unwind because my boss told me that unwind is a costly operation it will effect the app performance. so without using unwind I need to match these conditions. could you please help me out of this.

see this , its like the same question with same conditions and data, just you have 1 more condition and 1 more nested array. What do you mean sort the result for the user who is viewing the comment? — Takis
– Takis, Commented Mar 4, 2022 at 12:24

Wernfried Domscheit · Accepted Answer · 2022-03-04 12:38:51Z

First you can filter the documents which do not meet the conditions:

db.collection.aggregate([
  {
    "$match": {
      "data.videoId": 29,
      "data.videoComments.deletedbyowner": "FALSE",
      "data.videoComments.replyComments.replyDeletedbyowner": "FALSE"
    }
  }
])

I guess after this stage the majority of documents are filtered out and $unwind should not cause any performance issue anymore.

If the data is still too big, remove the videos:

db.collection.aggregate([
  {
    "$match": {
      "data.videoId": 29,
      "data.videoComments.deletedbyowner": "FALSE",
      "data.videoComments.replyComments.replyDeletedbyowner": "FALSE"
    }
  },
  {
    $set: {
      data: {
        $filter: {
          input: "$data",
          cond: { $eq: [ "$$this.videoId",  29 ] }
        }
      }
    }
  }
])

If the data is even then still too big, remove the comments:

db.collection.aggregate([
   {
      $match: {
         "data.videoId": 29,
         "data.videoComments.deletedbyowner": "FALSE",
         "data.videoComments.replyComments.replyDeletedbyowner": "FALSE"
      }
   },
  {
    $set: {
      data: {
        $filter: {
          input: "$data",
          cond: { $eq: [ "$$this.videoId",  29 ] }
        }
      }
    }
  },
   {
      $set: {
         data: {
            $map: {
               input: "$data",
               as: "video",
               in: {
                  $filter: {
                     input: "$$video.videoComments",
                     as: "comment",
                     cond: { $eq: ["$$comment.deletedbyowner", "FALSE"] }
                  }
               }
            }
         }
      }
   }
])

I think at this stage you have to use $unwind. Otherwise you need nested $filter and $map and the aggregation pipeline is getting complicated.

Latest when you need to sort the data you must use $unwind because you cannot sort elements in an array - this will be available in upcoming MongoDB release 5.2 with $sortArray

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