class User: Object {

    @objc dynamic var email: String = ""
    @objc dynamic var password: String = ""
    let subscriptions = List<Subscription>()

    convenience init(email: String, password: String) {
        self.init()

        self.email = email
        self.password = password
    }
}

class Subscription: Object {

    @objc dynamic var id: String?
    @objc dynamic var status: String?
    @objc dynamic var lastUpdated: Date?

    convenience init(id: String, status: String) {
        self.init()

        self.id = id
        self.status = status
        lastUpdated = Date()
    }
}

class func create(user: User, _ completion: (Error?) -> Void) {

        do {
            let ctx = try Realm()

            if ctx.isEmpty {
                try ctx.write {
                    ctx.add(user)

                    completion(nil)
                }
            } else {
                completion(nil)
            }
        } catch {
            completion(DBError.unableToAccessDatabase)
        }
    }

class func add(Subscription: Subscription, _ completion: (Error?) -> Void) {

        do {
            let ctx = try Realm()

            if let user = ctx.objects(User.self).last {

                let predicate = NSPredicate(format: "id = %@", subscription.id!)
                let sub = user.subscriptions.filter(predicate).first
                if sub == nil {
                    try ctx.write({
                        user.subscriptions.append(subscription)
                        completion(nil)
                    })
                }
            }
        } catch {
            completion(DBError.unableToAccessDatabase)
        }
    }