class ChallengeModel: Decodable {
    var author: AuthorModel?
    var categories: [CategoriesModel]?
    var created_at: UInt?
    var id: String?
    var opponent: AuthorModel?
    var questions: [QuestionModel]?

    required init?(json: JSON) {
        author = "author" <~~ json
        categories = "categories" <~~ json
        created_at = "created_at" <~~ json
        id = "id" <~~ json
        opponent = "opponent" <~~ json
        questions = "questions" <~~ json
…