MongoDB/Mongoose Get length of array attribute of object using aggregation

Question

 Problem.aggregate(
    [
      { $unwind: "$comments" },
      {
        $group: {
          _id: "$_id",
          size: { $size: "$comments" }
        }
      }
    ],
    function(err, results) {
      console.log(err);
      console.log(results);
    }
  );

I tried to use $size with aggregation, but I get this error:

MongoError: unknown group operator '$size'.

I am using mongodb version 3.6. I just want to get the length of the comments attribute which is an array.

Here is a sample problem document:

{
    "_id": {
        "$oid": "5e02a2184a6e7f4980f47e8f"
    },
    "author": {
        "id": {
            "$oid": "5e027a4badd90919304a9eb0"
        }
    },
    "description": "testd",
    "gender": 0,
    "dailyUpvotes": 2,
    "tags": [
        {
            "_id": {
                "$oid": "5e02a2184a6e7f4980f47e90"
            },
            "name": "test"
        }
    ],
    "title": "test",
    "upVotes": 2,
    "comments": [
        {
            "_id": {
                "$oid": "5e03124933c17d406471d6f9"
            },
            "id": {
                "$oid": "5e03124933c17d406471d6f8"
            }
        }
    ],
    "upVotesList": [
        {
            "_id": {
                "$oid": "5e045a07745eb94b584d27e1"
            },
            "userID": {
                "$oid": "5e045a00745eb94b584d27e0"
            }
        },
        {
            "_id": {
                "$oid": "5e02a26b3cbe4e3794555f42"
            },
            "userID": {
                "$oid": "5e027a4badd90919304a9eb0"
            }
        }
    ],
    "createdAt": {
        "$date": "2019-12-24T23:41:12.707Z"
    },
    "updatedAt": {
        "$date": "2019-12-26T06:58:15.080Z"
    },
    "__v": 2
}

Instead of getting the array of , I just want to get the length of the array. If there is another way that does not use aggregation, that would be fine too :) Thanks in advance!

Does this answer your question? [MongoDB: count the number of items in an array](https://stackoverflow.com/questions/21387969/mongodb-count-the-number-of-items-in-an-array) — whoami - fakeFaceTrueSoul, Dec 27 '19 at 03:59

score 1 · Accepted Answer · answered Dec 27 '19 at 03:03

1

Try something like this.

Problem.aggregate([
   {
      $project: {
         item: 1,
         numberOfResults: { $size: "$comments" }
      }
   }
] )

take alook at https://docs.mongodb.com/manual/reference/operator/aggregation/size/

answered Dec 27 '19 at 03:03

RamPrakash

1,687
3
20
25

MongoDB/Mongoose Get length of array attribute of object using aggregation

1 Answers1