您只需要自己实现 init(from:)并尝试解码 subscription 的值键既作为表示 Subs 的 Dictionary ，又作为 String 的键.

You simply need to implement init(from:) yourself and try decoding the value for the subscription key both as a Dictionary representing Subs and as a String.

struct OrderLine: Codable {
    let absUrl: String?
    let restApiUrl : String?
    let description : String?
    let quantity : Int?
    let subscription: Subs?
    let total: Double?

    private enum CodingKeys: String, CodingKey {
        case absUrl, restApiUrl, description, quantity, subscription, total
    }

    init(from decoder:Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.absUrl = try container.decodeIfPresent(String.self, forKey: .absUrl)
        self.restApiUrl = try container.decodeIfPresent(String.self, forKey: .restApiUrl)
        self.description = try container.decodeIfPresent(String.self, forKey: .description)
        self.quantity = try container.decodeIfPresent(Int.self, forKey: .quantity)
        self.total = try container.decodeIfPresent(Double.self, forKey: .total)
        if (try? container.decodeIfPresent(String.self, forKey: .subscription)) == nil {
            self.subscription = try container.decodeIfPresent(Subs.self, forKey: .subscription)
        } else {
            self.subscription = nil
        }
    }
}