我想在这个结构中添加一个isFavourite变量。当我添加这个时，JSON解码会给出错误。如何管理JSON文件中的外部变量？

struct ProductModel : Decodable {
    let productData : [ProductData]
    
    private enum CodingKeys:String,CodingKey {
        case productData = "data"
    }
}

struct ProductData : Identifiable,Decodable,Hashable {
    var id : Int
    var title :String
    var type : String
    var description : String
    var filename : String
    var price : Float
    var rating : Int
    //var imageName : String? = "vegetable"
    
    init() {
        self.id = 0
        self.title = ""
        self.type = ""
        self.description = ""
        self.filename = ""
        self.price = 0.0
        self.rating = 0
    }
}