MongoDB $lookup array of lookup field values

I’m using MongoDB, and I tried to do it with many pipeline steps but I couldn’t find a way.

I have a players collection, each player contains an items array

{
    "_id": ObjectId("5fba17c1c4566e57fafdcd7e"),
    "username": "moshe",
    "items": [ 
        "5fbb5ab778045a985690b5fc",
        "5fbb5ab778045a985690b5fd"
    ]
}

I have an items collection where there is more information about each item in the player items array.

{
    "_id": ObjectId("5fbb5ab778045a985690b5fc"),
    "name": "Axe",
    "damage": 4,
    "defense": 6
}
{
    "_id": ObjectId("5fbb5ab778045a985690b5fd"),
    "name": "Demo",
    "damage": 1,
    "defense": 1
}

My goal is to have a player document with all the information about the item inside his items array, so it will look like that:

{
    "_id": ObjectId("5fba17c1c4566e57fafdcd7e"),
    "username": "moshe",
    "items": [
        {
            "_id": ObjectId("5fbb5ac178045a985690b5fd"),
            "equipped": false,
            "itemId": "5fbb5ab778045a985690b5fc",
            "name": "Axe",
            "damage": 4,
            "defense": 6
        },
        {
            "_id": ObjectId("5fbb5ac178045a985690b5fa"),
            "equipped": false,
            "itemId": "5fbb5ab778045a985690b5fd",
            "name": "Demo",
            "damage": 1,
            "defense": 1
        }
    ]
}

>Solution :

One option is to use $lookup with a pipeline to overcome the difference between ObjectId and string:

db.players.aggregate([
  {$lookup: {
      from: "items",
      let: {items: "$items"},
      pipeline: [
        {$match: {$expr: {$in: [{$toString: "$_id"}, "$$items"]}}}
      ],
      as: "items"
    }
  }
])

See how it works on the playground example