flask API mongoDB 'NoneType' object has no attribute 'user'

I have no problem with MongoCompass (localhost), but when i am trying to make everything with Mongo Atlas (to deploy then in heroku), I have error:

line 61, in post
    if len(list(mongo.db.user.find({'studentId': args['studentId']}))) == 0:
AttributeError: 'NoneType' object has no attribute 'user'

my code:

class User(Resource):
    def post(self):
        args = user_put_args.parse_args()
        if len(list(mongo.db.user.find({'studentId': args['studentId']}))) == 0:
            args['events'] = []
            organisers_f = open(f'{os.path.dirname(os.path.abspath(__file__))}/organisers.txt', 'r')
            organisers = [l.strip() for l in organisers_f.readlines()]
            args['permission'] = 0

            if args['studentId'] in organisers:
                args['permission'] = 1
                args['organisedEvents'] = []

            result = mongo.db.user.insert_one(args)
            print(result.inserted_id)
            return {'id': str(result.inserted_id), 'permission': args['permission']}