mongodb aggregate flatten resultat

db.collection.aggregate([
              ....
    { $group: {_id: "$data.r.id", "col": {$first: "$data.r"}}},
    {$project:{"o":{"$objectToArray":"$col"}}},
    {$unwind:"$o"},
    {$group:{"_id":null, "keys":{$addToSet:"$o.k"}}},
    {$project: {"keys":1, "_id": 0}},
    {$addFields: {res:{$map:{ input:"$keys", as: "a", in: {"_id":0,"label": "$$a"}}}}},
    {$project: {"res":1}}
])

With my mongodb query the output is:

{ 
"res" : [
    {
        "_id" : 0.0, 
        "label" : "aaaa"
    }, 
    {
        "_id" : 0.0, 
        "label" : "bbbbb"
    }, 
    {
        "_id" : 0.0, 
        "label" : "ccccc"
    }
] }

and i try to output this

        {
            "_id" : 0.0, 
            "label" : "aaaa"
        }, 
        {
            "_id" : 0.0, 
            "label" : "bbbb"
        }, 
        {
            "_id" : 0.0, 
            "label" : "cccc"
        }
    

so i added

{$replaceRoot : {"newRoot": "$res"}}

but i got this error

”newRoot’ expression must evaluate to an object, but resulting value
was: [{_id: 0, label: "aaaa"},{_id: 0, label: "bbbb"},{_id: 0, label:
"cccc"}]

How can i remove the uselesss nested object "res" to return only the list/array of object(id,label) ?

>Solution :

Re-write the answer from the comment to resolve the question.

You need to flatten the deconstruct res array first

{ $unwind: "$res" }

before replacing the input documents with

{
  $replaceRoot: {
    newRoot: "$res"
  }
}