import UIKit

let playersResponse = """
[
    {
        "id": 1,
        "name": "John Doe"
    },
    {
        "id": 15,
        "name": "Jessica Thump"
    }
]
"""

struct Player: Decodable {
    var id: Int
    var name: String?
}

let players = try! JSONDecoder().decode([Player].self, from: playersResponse.data(using: .utf8)!)

let gamesResponse = """
[
    {
        "name": "Tic Tac Toe",
        "playerId1": 15,
        "playerId2": 1
    }
]
"""

struct Game: Decodable {
    var name: String
    var player1: Player
    var player2: Player

    enum CodingKeys: String, CodingKey {
        case name
        case player1 = "playerId1"
        case player2 = "playerId2"
    }
}

extension Game {

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)

        name = try values.decode(String.self, forKey: .name)

        //                                             HOW SHOULD I RETRIEVE THE PLAYER'S NAME GIVEN THEIR ID HERE?
        //                                                                         |
        //                                                                         |
        //                                                                         V
        player1 = Player(id: try values.decode(Int.self, forKey: .player1), name: nil)
        player2 = Player(id: try values.decode(Int.self, forKey: .player2), name: nil)
    }
}

let games = try! JSONDecoder().decode([Game].self, from: gamesResponse.data(using: .utf8)!)
dump(games[0].player1)