Convert ObjectId to String and vice versa inside MongoDB aggregate map

I have the following document in a collection:

Input:

{
    "_id" : ObjectId("***"),
    "oldItems" : [ 
        {
            "_id" : ObjectId("***"),
            "name" : "ItemId***",
            "nestedItemsToExtract" : { }
        }
    ]
}

      

        
        
        
      

    

I need to iterate through an array oldItems

and create a resulting output document with another array whose values ​​are to be matched against the original.

Output:

{
    "_id" : ObjectId("***"),
    "newItems" : [ 
        {
            "oldItemId" : "***", // String Value Of Parent / Mapped Item Id aka ObjectId.toString()
            "_id" : ObjectId("***") // New ObjectId Here aka ObjectId()
        }
    ]
}

      

        
        
        
      

    

How can I convert the mapped (old) _id

ObjectId value to String, but generate a new ObjectId for the (new) property _id

in the resulting array?

Edit:

I was able to find a workaround to create a new ObjectId value using an expression "{ $literal: ObjectId() }"

. I have adjusted my Exit code snippet:

use DB_NAME_HERE

db.getCollection('COLLECTION_NAME_HERE').aggregate([
    { $match: {} },
    { $project: {
        newItems: {
            $map: {
                input: '$oldItems',
                as: 'oldItem',
                in: {
                    oldItemId: '$$oldItem._id' // Returns ObjectId
                    _id: ObjectId() // Fails With 'disallowed field type OID in object expression (at '_id')"'
                    //Edit
                    //_id: { $literal: ObjectId() } // Works
                }
            }
        }       
    }
}])