MongoDB $lookup on array of objects

Categories

{
    "_id" : ObjectId("61740086893f048528d166b9"),
    "name": "Category1",
    "tracks" : [ 
        "61c65353565a2d9a1cd3020d", 
        "61c74518962dc3efb96c3438", 
        "61c74775703176a6f72df444"
    ]
}

Tracks

{
    "_id" : ObjectId("61c65353565a2d9a1cd3020d"),
    "name" : "Track1",
    "categoryId" : ObjectId("61740086893f048528d166b9"),
    "creatorId" : ObjectId("61c6478304e98ed63e8ee7d3"),
    "thumbnailId" : ObjectId("61c65353565a2d9a1cd3020c"),
    "plays" : [],
    "media" : {
        "type" : "wav",
        "url" : ""
    },
    "status" : "approved",
    "downloads" : [],
    "uploadedDate" : 1640387411
}

Assuming that I have 5 categories and each category has many tracks ID, I wanna get N last tracks for each category so I used this code below

categories.aggregate([
    {
        $project: {
            tracks: { $slice: ["$tracks", -2] },
        },
    },
]

And the response is

[
    {
        "_id": "61740086893f048528d166b9",
        "tracks": [
            "61c74518962dc3efb96c3438",
            "61c74775703176a6f72df444"
        ]
    },
    {
        "_id": "61740094893f048528d166c1",
        "tracks": []
    },
    {
        "_id": "617400a0893f048528d166cb",
        "tracks": []
    }
]

So far it’s good, but the question is how can I replace each category’s tracks from an array of IDs to an array of objects?

I tried $loopup but I probably didn’t implement the localField correctly.

Expected result

[
    {
        "_id": "61740086893f048528d166b9",
        "tracks": [
            {
                "_id": ObjectId("61c74518962dc3efb96c3438")
                ...
            },
            {
                "_id": ObjectId("61c74775703176a6f72df444")
                ...
            }
        ]
    },
    {
        "_id": "61740094893f048528d166c1",
        "tracks": []
    },
    {
        "_id": "617400a0893f048528d166cb",
        "tracks": []
    }
]

>Solution :

$lookup is the correct way for you to find the corresponding object in Tracks collection. Why your code does not work is that you are storing strings in tracks array in Categories collection; while the _id of Tracks collection is ObjectId. There will be no $lookup result as the datatypes do not match. What you can do is converting the strings to ObjectId by using $toObjectId in a $map, and then do the $lookup

db.categories.aggregate([
  {
    $project: {
      tracks: {
        $slice: [
          "$tracks",
          -2
        ]
      }
    }
  },
  {
    $project: {
      tracks: {
        "$map": {
          "input": "$tracks",
          "as": "t",
          "in": {
            "$toObjectId": "$$t"
          }
        }
      }
    }
  },
  {
    "$lookup": {
      "from": "tracks",
      let: {
        t: "$tracks"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              "$in": [
                "$_id",
                "$$t"
              ]
            }
          }
        }
      ],
      "as": "tracks"
    }
  }
])

Here is the Mongo playground for your reference.