Flutter JSON serializable using json_annotation package how to use JsonConverter with Firebase Firestore DocumentReference

Question

I'm trying to save a DocumentReference in models using JSON serializable with a custom JsonConverter but not doing it correct

here is my model

@DocumentSerializer()
DocumentReference? recentTrainingRef;

My DocumentSerializer Class

class DocumentSerializer
    implements JsonConverter<DocumentReference, DocumentReference> {
  const DocumentSerializer();

  @override
  DocumentReference fromJson(DocumentReference docRef) => docRef;

  @override
  DocumentReference toJson(DocumentReference docRef) => docRef;
}

I'm getting the following error Could not generate fromJson code for recentTrainingRef.