How to add length of one array field into another field in Mongodb document

Question

I am a newbie in Mongodb. My Mongodb version is v4.4.2. I create a collection in Mongo and want to update one record each time. I receive this record and search collection, if the record is in the collection before, I update that. Unless, I insert the record.

dic = {"CST_NUM": "7101",
       "CNT_TRX_WEEK": 0.224,
       "cities": {"G": {"t": 6, "f": 0},
                  "K": {"t": 4, "f": 0}, "S": {"t": 1, "f": 0}},
       "cst_trm": ["123", "342"]
       }

Also, each record has an array field that when it updated, I must add the size of the array field into another field of the record. Its name is "cst_trm_cnt". I update the collection with this code:

  mycollection.update_one(
 {"CST_NUM": dic["CST_NUM"]},
        {
            "$inc": {"CNT_TRX_WEEK": +dic["CNT_TRX_WEEK"],"cities.S.t": +dic["cities"]["S"]["t"]},
         "$addToSet": { "cst_trm": {"$each":dic["cst_trm"]}},
            "$addfield": {"cst_trm_cnt": {"$size":"cst_trm"}}
        }
    )  

But, I receive this error in the last line of the above code:

 pymongo.errors.WriteError: Unknown modifier: $addfield. Expected a valid update modifier or pipeline-style update specified as an array, full error: {'index': 0, 'code': 9, 'errmsg': 'Unknown modifier: $addfield. Expected a valid update modifier or pipeline-style update specified as an array'}

Would you please guide me how to add length of the array into another field of the record in Mongo?

Any help is really appreciated.

Answer 1

Problem solved. After Updating, in the code, I must add name of the collection, which I have already made,in $out part,unless Mongo builds a new collection. The code is in following:

 mytable.aggregate([
             {"$addFields": {"cst_trm_cnt":{"$size":"$cst_trm"}}},
             {"$out": "test"}
                   ])