Lookup inside first field of array of objects MONGO

I have a dataset in which i want to put lookup inside an id that is inside an array of objects . now when i add lookup . it replaces the complete document with the lookup result . Now inside that object i have some other content which i want to access, but it gets replaced. Also i dont want to put lookup in all elements of that array of objects just the first one. The main this is i want to access scale after the lookup is done

Below is my data-set

"user_id" : ObjectId("5b17b992c440782b5a218cd2"),
        "commission" : [
                {
                        "commissson_id" : ObjectId("5bc05984b49e14243f2973ad"),
                        "scale" : "14"
                },
                {
                        "commissson_id" : ObjectId("5bc05992b49e14243f2973ae"),
                        "scale" : "14"
                },
                {
                        "commissson_id" : ObjectId("5bc05984b49e14243f2973ad"),
                        "scale" : "1"
                },
                {
                        "commissson_id" : ObjectId("5bc0596a414d30243f94b0de"),
                        "scale" : "1"
                }
             ]
             

So far i have tried this

$pipe=[
                        [
                                '$match'=>
                                                [
                                                        '_id'=> $user_id
                                                ],
                        ],
                        [
                                '$lookup'=>
                                                [
                                                        'from'=>'commission',
                                                        'localField'=>'commission.0.commissson_id',
                                                        'foreignField'=> '_id',
                                                        'as'=> 'commission'
                                                ]
                        ],
        ];