flat JSON to struct with nested properties

Question

All questions I have found so far on the searches I've done is about decoding nested JSON to some struct with nested properties. I want to do the opposite: decode flat JSON to a struct with nested properties.

Here's example JSON:

{
    "id":"ABC123",
    "cell":"test",
    "qty":24
}

which I'd like to decode to this struct:

struct InventoryItem {
    let id: String
    let mfgInfo: MfgInfo
}

extension InventoryItem {
    struct MfgInfo {
        let cell: String
        let qty: Int
    }
}

I have tried adding CodingKeys for each struct:

struct InventoryItem: Decodable {
    let id: String
    let mfgInfo: MfgInfo

    enum CodingKeys: String, CodingKey {
        case id, mfgInfo
    }
}


struct MfgInfo: Decodable {
    let cell: String
    let qty: Int

    enum CodingKeys: String, CodingKey {
        case cell, qty
    }
}

But this doesn't work. I get this error:

No value associated with key CodingKeys(stringValue: \"mfgInfo\", intValue: nil) (\"mfgInfo\"), converted to mfg_info.

How can I make this work without a custom initializer? Or do I need to write a custom init(with: Decoder) initializer?

There is a full example for this in the documentation provided by Apple: developer.apple.com/documentation/foundation/… under "Encode and Decode Manually"! — KevinP
– KevinP, Commented Mar 9, 2020 at 20:01
@Mamaessen, I read that documentation, but it appears to deal with encoding nested JSON to a flat struct. I'm trying to do the opposite (flat JSON to nested struct). — gnarlybracket
– gnarlybracket, Commented Mar 9, 2020 at 20:03

gnarlybracket · Accepted Answer · 2020-03-09 20:43:01Z

1

I wasn't able to do this without implementing the init(from: Decoder) initializer, but it was easier than I thought. For nested structs that just have primitive types, you can just use the automatically synthesized init(from: Decoder) method.

Final struct setup:

struct InventoryItem: Decodable {
    let id: String
    let mfgInfo: MfgInfo

    enum CodingKeys: String, CodingKey {
        case id
    }

    struct MfgInfo: Decodable {
        let cell: String
        let qty: Int

        enum CodingKeys: String, CodingKey {
            case cell, qty
        }
    }

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

        id = try container.decode(String.self, forKey: .id)
        mfgInfo = try MfgInfo(from: decoder)
    }
}

answered Mar 9, 2020 at 20:43

gnarlybracket

1,7154 gold badges19 silver badges39 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Gereon · Accepted Answer · 2020-03-09 19:59:11Z

0

This is only possible with a custom init(with: Decoder) implementation, or by having e.g. a FlatInventoryItem type that conforms to Decodable and then providing conversion methods between that and your desired InventoryItem type.

answered Mar 9, 2020 at 19:59

Gereon

18k4 gold badges46 silver badges80 bronze badges

1 Comment

gnarlybracket Over a year ago

This is kinda what I thought, but I was hoping there was a more concise way.

Collectives™ on Stack Overflow

flat JSON to struct with nested properties

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related