[cloud_firestore_odm] How do I add DocumentReference parameters to my model?

[ywake]

I want to make a model like this.

@freezed
class Tweet with _$Tweet {
  const factory Tweet(
    @Id() required String id,
    required String tweet,
    required User author, // <- I want to make this a DocumentReference.
    required DocumentReference<Map<String, dynamic>> author, // <- "Could not generate `fromJson` code for `author`. To support the type DocumentReference"
  ) = _Tweet;
}

[ywake]

I found this
#7475 (reply in thread)

UserDocumentReference docRefFromJson(String id) => usersRef.doc(id);
String docRefToJson(UserDocumentReference ref) => ref.id;

@freezed
class Tweet with _$Tweet {
  const factory Tweet(
    @Id() required String id,
    required String tweet,
    @JsonKey(fromJson: docRefFromJson, toJson: docRefToJson)
    required UserDocumentReference author,
  ) = _Tweet;
}

but got error

type '_JsonDocumentReference' is not a subtype of type 'String' in type cast

// ...
_$_Task _$$_TaskFromJson(Map<String, dynamic> json) => _$_Task(
      id: json['id'] as String,
      tweet: json['tweet'] as String,
      author: docRefFromJson(json['from'] as String), // <-Error occurred here
// ...