struct Contact: Decodable, CustomStringConvertible {
    var id: String
    @NestedKey
    var firstname: String
    @NestedKey
    var lastname: String
    @NestedKey
    var address: String

    enum CodingKeys: String, NestableCodingKey {
        case id
        case firstname = "nested/data/user/firstname"
        case lastname = "nested/data/user/lastname"
        case address = "nested/data/address"
    }

    var description: String {
        "Contact(firstname: \(firstname), lastname: \(lastname), address: \(address))"
    }
}


let json = """
[
  {
    "id": "1",
    "nested": { "data": {
      "user": { "firstname": "Alice", "lastname": "Wonderland" },
      "address": "Through the looking glass"
    } }
  },
  {
    "id": "2",
    "nested": { "data": {
      "user": { "firstname": "Bob", "lastname": "Builder" },
      "address": "1, NewRoad"
    } }
  }
]
""".data(using: .utf8)!

let decoder = JSONDecoder()
let list = try decoder.decode([Contact].self, from: json)
// [Contact(firstname: Alice, lastname: Wonderland, address: Through the looking glass), Contact(firstname: Bob, lastname: Builder, address: 1, NewRoad)]