Codable inheritence

Question

I have three classes:

class User : NSObject, Codable{

    var id: Int! = 0
    var userName: String!

   enum CodingKeys:String,CodingKey{
      case id
      case userName = "user_name"
   }

}

class SpecialUser : User{
    var isAdmin: Bool!    
}

class BlockedUser : User{
    var isBlocked: Bool!
}

When I try to decode Special user or BlockedUser their extra properties always come up null How do I decode SpecialUser and BlockedUser correctly?