non-matching-keys.swift

import Foundation

let json = """
{
  "userId": 0,
  "name": "Donny"
}
""".data(using: .utf8)!

struct User: Codable {
  enum DecodingKeys: CodingKey {
    case userId, name
  }
  
  enum EncodingKeys: String, CodingKey {
    case userId = "user_id"
    case name
  }
  
  let userId: Int
  let name: String
  
  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: DecodingKeys.self)
    self.userId = try container.decode(Int.self, forKey: .userId)
    self.name = try container.decode(String.self, forKey: .name)
  }
  
  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: EncodingKeys.self)
    try container.encode(userId, forKey: .userId)
    try container.encode(name, forKey: .name)
  }
}

let decoder = JSONDecoder()
let encoder = JSONEncoder()

let user = try! decoder.decode(User.self, from: json)
let data = try! encoder.encode(user)

// prints "{\"user_id\":0,\"name\":\"Donny\"}"
print(String(data: data, encoding: .utf8)!)