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