Swift 4 Decodable multiple containers - json

Question

I'm trying to understand how could I parse this multiple container JSON to an object. I've tried this approach (Mark answer), but he explain how to solve it using one-level container. For some reason I can't mimic the behaviour for multiple containers.
{
"graphql": {
"shortcode_media": {
"id": "1657677004214306744",
"shortcode": "BcBQHPchwe4"
}
}
}
class Post: Decodable {
enum CodingKeys: String, CodingKey {
case graphql // The top level "user" key
case shortcode_media
}
enum PostKeys: String, CodingKey {
case id
}
required init(from decoder: Decoder) throws {
let values = try decoder.container(keyedBy: CodingKeys.self)
let post = try values.nestedContainer(keyedBy: PostKeys.self, forKey: .shortcode_media)
self.id = try post.decode(String.self, forKey: .id)
}
var id: String
}
I'm getting:
Swift.DecodingError.Context(codingPath: [], debugDescription: "Cannot get KeyedDecodingContainer<PostKeys> -- no value found for key \"shortcode_media\"", underlyingError: nil))
Any help will be much appreciated, thank you!

Answer 1

As vadian notes, you haven't matched the JSON structure. There is no shortcode_media key at the top level like you've encoded in CodingKeys.
In order to decode this with a custom decoder, you will need to walk through each level and deal with it.
class Post: Decodable {
enum CodingKeys: String, CodingKey {
case graphql
}
enum GraphQLKeys: String, CodingKey {
case shortcode_media
}
enum PostKeys: String, CodingKey {
case id
}
required init(from decoder: Decoder) throws {
// unload the top level
let container = try decoder.container(keyedBy: CodingKeys.self)
// Unload the graphql key
let graphql = try container.nestedContainer(keyedBy: GraphQLKeys.self, forKey: .graphql)
// unload the shortcode_media key
let post = try graphql.nestedContainer(keyedBy: PostKeys.self, forKey: .shortcode_media)
// Finally, unload the actual object
self.id = try post.decode(String.self, forKey: .id)
}
var id: String
}

Answer 2

Please read the JSON.
Any opening { is quasi a separator. The indentation of the JSON indicates also the hierarchy.
For clarity I removed all coding keys and left the variable names – which should be camelCased – unchanged.
struct Root : Decodable {
let graphql : Graph
// to access the `Media` object declare a lazy instantiated property
lazy var media : Media = {
return graphql.shortcode_media
}()
}
struct Graph : Decodable {
let shortcode_media : Media
}
struct Media : Decodable {
let id: String
let shortcode : String
}
let jsonString = """
{
"graphql": {
"shortcode_media": {
"id": "1657677004214306744",
"shortcode": "BcBQHPchwe4"
}
}
}
"""
do {
let data = Data(jsonString.utf8)
var result = try decoder.decode(Root.self, from: data)
print(result.media)
} catch {
print("error: ", error)
}
Writing a custom initializer with nestedContainer is more effort than creating the actual hierarchy.
Please paste the entire code in a Playground and check it out.