Find Index of Object in array with Aggregate

Question

Is there a way to get index in aggregate pipeline, I have a result from long aggreagte query

[
    {
        "_id": "59ed949227ec482044b2671e",
        "points": 300,
        "fan_detail": [
            {
                "_id": "59ed949227ec482044b2671e",
                "name": "mila   ",
                "email": "[email protected] ",
                "password": "$2a$10$J0.KfwVnZkaimxj/BiqGW.D40qXhvrDA952VV8x.xdefjNADaxnSW",
                "username": "mila  0321",
                "updated_at": "2017-10-23T07:04:50.004Z",
                "created_at": "2017-10-23T07:04:50.004Z",
                "celebrity_request_status": 0,
                "push_notification": [],
                "fan_array": [],
                "fanLength": 0,
                "celeb_bio": null,
                "is_admin": 0,
                "is_blocked": 2,
                "notification_setting": [
                    1,
                    2,
                    3,
                    4,
                    5,
                    6,
                    7
                ],
                "total_stars": 0,
                "total_points": 134800,
                "user_type": 2,
                "poster_pic": null,
                "profile_pic": "1508742289662.jpg",
                "facebook_id": "alistnvU79vcc81PLW9o",
                "is_user_active": 1,
                "is_username_selected": "false",
                "__v": 0
            }
        ]
    }
],

so I want to find the index of _id in aggregate query and above array can contain 100s of object in it.

Neil Lunn · Accepted Answer · 2017-11-13 10:34:25Z

6

Depending on the available version of MongoDB you have there are different approaches:

`$indexOfArray` - MongoDB 3.4

The best operator for this is simply $indexOfArray where you have it available. The name says it all really:

Model.aggregate([
  { "$match": { "fan_detail._id": mongoose.Types.ObjectId("59ed949227ec482044b2671e") } },

  { "$addFields": { 
    "fanIndex": {
      "$indexOfArray": [
        "$fan_detail._id",
        mongoose.Types.ObjectId("59ed949227ec482044b2671e")
      ]
    }
  }}
])

`$unwind` with `includeArrayIndex` - MongoDB 3.2

Going back a version in releases, you can get the index from the array from the syntax of $unwind. But this does require you to $unwind the array:

Model.aggregate([
  { "$match": { "fan_detail._id": mongoose.Types.ObjectId("59ed949227ec482044b2671e") } },
  { "$unwind": { "path": "$fan_detail", "includeArrayIndex": true } },
  { "$match": { "fan_detail._id": mongoose.Types.ObjectId("59ed949227ec482044b2671e") } }
])

mapReduce - Earlier versions

Earlier versions of MongoDB to 3.2 don't have a way of returning an array index in an aggregation pipeline. So if you want the matched index instead of all the data, then you use mapReduce instead:

Model.mapReduce({
  map: function() {
    emit(
      this._id,
      this['fan_detail']
        .map( f => f._id.valueOf() )
        .indexOf("59ed949227ec482044b2671e") 
    )
  },
  reduce: function() {},
  query: { "fan_detail._id": mongoose.Types.ObjectId("59ed949227ec482044b2671e") }
})

In all cases we essentially "query" for the presence of the element "somewhere" in the array beforehand. The "indexOf" variants would return -1 where nothing was found otherwise.

Also $addFields is here just for example. If it's your real intent to not return the array of 100's of items, then you're probably using $project or other output anyway.

answered Nov 13, 2017 at 10:34

Neil Lunn

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2 Comments

Irfan Khan Over a year ago

Thanks for the answer and its correct yeah I am using pagination as well to limit the 100s records

Neil Lunn Over a year ago

@IrfanKhan Well there is also $slice if you were just simply looking to do that. "Pagination of arrays" is why that has been around since very early releases of MongoDB.

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Find Index of Object in array with Aggregate

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`$indexOfArray` - MongoDB 3.4

`$unwind` with `includeArrayIndex` - MongoDB 3.2

mapReduce - Earlier versions

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$indexOfArray - MongoDB 3.4

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`$indexOfArray` - MongoDB 3.4

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