extension User {
    func toJSON() -> [String : Any]
        return [
            "firstName": firstName,
            "lastName": lastName,
            "age": age,
            "friends": friends.map { $0.toJSON() },
            "coworkers": coworkers.map { $0.toJSON() },
            "favorites": favorites.map { $0.toJSON() },
            "messages": messages.map { $0.toJSON() },
            "notes": notes.map { $0.toJSON() },
            "tasks": tasks.map { $0.toJSON() },
            "imageURLs": imageURLs.map { $0.absoluteString },
            "groups": groups.map { $0.toJSON() }
        ]
    }
}