Firebase toObject with ref

With cloud firestore, you can convert a document to your object with document.toObject(YourClass.class); where the class's variables match those in your database. However, if one of the variables in the database is a reference, which data type would you use in the java class? Note that I need to not only store it in my data model but retrieve it and set it with the override methods of the form:

protected MyDataModel(Parcel in) {
    mString = in.readString();
}

and

@Override
public void writeToParcel(Parcel parcel, int i) {
    parcel.writeString(mString);
}

If I use a DocumentReference, then in writeToParcel(), how would I write it? Also, how would I read it in the protected Parcel in constructor? I have tried writeSerializable() and readSerializable() but DocumentReference does not implement Serializable.

from Firebase toObject with ref