Decoding numerical snake_case keys with JSONDecoder - swift

Question

I have the following JSON object which is to be converted to an object using JSONDecoder:
{
"first_key": 3,
"image_1000x1000": "location"
}
This maps to the following Swift model:
struct Response: Decodable {
var firstKey: Int
var image1000x1000: String
}
By using JSONDecoder with the .convertFromSnakeCase option, the snake_case keys inside the JSON are transformed to camelCase by using the algorithm defined in the documentation:
This strategy follows these steps to convert JSON keys to camel-case:
Capitalize each word that follows an underscore.
Remove all underscores that aren't at the very start or end of the string.
Combine the words into a single string.
Therefore, in this case:
first_key becomes firstKey (as expected)
image_1000x1000 should become image1000x1000
However when attempting to decode this response, a keyNotFound error for the image1000x1000 key is thrown (see this live example):
let json = "{\"first_key\": 3, \"image_1000x1000\": \"location\"}".data(using: .utf8)!
do {
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
let response = try decoder.decode(Response.self, from: json)
print(response)
} catch let e {
print(e)
}
What is incorrect about my camel case conversion of image_1000x1000, and why can’t JSONDecoder find the corresponding key?

Answer 1

You can see what the algorithm is expecting by running the process in reverse; use a JSONEncoder to encoder your data and inspect the output:
struct Response: Codable {
var firstKey: Int
var image1000x1000: String
}
let test = Response(firstKey: 10, image1000x1000: "secondKey" )
let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .convertToSnakeCase
let data = try encoder.encode(test)
print(String(data: data, encoding: .utf8)!)
This will produce:
{"first_key":10,"image1000x1000":"secondKey"}
So if you have control over the JSON and can live with image1000x1000 as a key, then you're done. If not you'll have to do something like this:
struct Response: Codable {
var firstKey: Int
var image1000x1000: String
private enum CodingKeys: String, CodingKey {
case image1000x1000 = "image_1000x1000"
case firstKey = "first_key"
}
}
Another option is to implement a custom key encoding strategy. It may end up being less code. See KeyEncodingStrategy for more about this.

Answer 2

I am facing the same issue my json keys are iso639_1 and iso639_2. Without adding Coding key it will works.
make your variable optional
Here it's my Decodable model
struct CountryModel: Decodable{
var name: String
var borders: [String]
var region: String
var alpha2Code: String
var alpha3Code: String
var flag: String
var languages: [languages]
}
struct languages: Decodable {
var iso639_1: String?
var iso639_2: String?
var name: String
var nativeName: String
}
When I add optional to both underscore variables iso639_1 and iso639_2. Then it works fine may be due to Null Value!
Here, in your case add optional to your image1000x1000 variable. Like below
struct Response: Decodable {
var firstKey: Int
var image_1000x1000: String?
}
Hope it will works!