I need help to understand DBRef in Flutter Schemas

{"$ref": <value>, "$id" : <value>, "$db" : <value>}

I’m creating a Flutter schema, but I don’t understand how to create a class to map a DBRef.
I’ve tried create a schema like this:

class _DBRef {
  late ObjectId? id;

But this field aways returns null. Like objectId and Uuid they are classes that I can use on schemas:

  late ObjectId id;

Is there a away to map a DBRef in a Flutter Schema? What I’m missing?