addField.dart

  Future<void> addField() {
    return collectionRef
        .doc('MyDoc')
        //will edit the doc if already available or will create a new doc with this given ID
        .set(
          {'role': "developer"},
          SetOptions(merge: true),
          // if set to 'false', then only these given fields will be added to that doc
        )
        .then((value) => debugPrint("User Added"))
        .catchError((error) => debugPrint("Failed to add user: $error"));
  }