Realm LinkingObjects not working. return nil

Question

So I am trying to work with LinkingObjects in Realm and seem to be getting bad results on the first level (linking not working).

I am going to add here all the code. Obviously not expecting people to go over it all, just want to show that I went over all answers that I saw so far in Stack Overflow.

class Book: Object {
    @objc dynamic var bookId = 0
    @objc dynamic var bookName = ""
    var sections = List<Section>()

    override static func primaryKey() -> String? {
        return "bookId"
    }
}

class Section: Object {
    @objc dynamic var sectionId = 0
    @objc dynamic var sectionName = ""
    let chapters = List<RChapter>()
    let book = LinkingObjects(fromType: Book.self, property: "sections")

    override static func primaryKey() -> String? {
        return "sectionId"
    }
}

class RChapter: Object {
    @objc dynamic var chapterId = 0
    @objc dynamic var chapterName = ""
    var section = LinkingObjects(fromType: Section.self, property: "chapters")

    override static func primaryKey() -> String? {
        return "chapterId"
    }
}

class DataBase {
    static func getChapterById(id: String) -> RChapter? {
        let realm = try! Realm()
        let chapter = realm.objects(RChapter.self).filter("chapterId = \(id)").first
        return chapter
    }

    static func writeToRealm(json: JSON) {
        let book = Book()
        book.bookId = json["bookId"].int!
        book.bookName = json["bookName"].string!
        let section = Section()
        section.sectionId = json["sectionId"].int!
        section.sectionName = json["sectionName"].string!
        let chapter = RChapter()
        chapter.chapterId = json["chapterId"].int!

        section.chapters.append(chapter)
        book.sections.append(section)

        let realm = try! Realm()
        try! realm.write {
            realm.add(book, update: true)
        }
    }
}

but after filling up the database I cannot get section from the chapter.

anyone know why? Thanks,