Trouble decoding JSON using the Swift Decoder protocol

Question

I'm trying to use the Swift Decoder protocol to parse some JSON, but I'm getting errors that I can't make sense of. The JSON I'm getting back is problematic because not all of the keys are certain to be present, so I've given the properties of the corresponding struct default values and setup an initializer, but I suspect my approach is wrong.

In the "foods" array, some of the items have the property "ndbNumber" (String), and some have fdcId (Int) and gtinUpc (String) properties. Otherwise the items are identical and represent food items returned from a search.

This is the struct for the outermost JSON level.

public struct ReturnedFoods: Decodable {
    public let count: Int
    public let all: [ReturnedFood]

    enum CodingKeys: String, CodingKey {
        case count = "totalHits"
        case all = "foods"
    }
}

And this is for the the next level where the food items are:

public struct ReturnedFood: Codable {

    public var ndbNumber: String = ""
    public var fdcId : Int = -1
    public var description: String = ""
    public var dataType : String = ""
    public var brandOwner: String = ""
    public var gtinUpc : String = ""
    public var ingredients : String = ""
    public var publishedDate : String = ""
    public var score : Float = -1

    public init(from decoder: Decoder) throws  {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        if let ndbNumber = try values.decodeIfPresent(String.self, forKey: .ndbNumber) {
            self.ndbNumber = ndbNumber
        }
        if let fdcId = try values.decodeIfPresent(Int.self, forKey: .ndbNumber) {
            self.fdcId = fdcId
        }
        if let description = try values.decodeIfPresent(String.self, forKey: .description) {
            self.description = description
        }
        if let dataType = try values.decodeIfPresent(String.self, forKey: .dataType) {
            self.dataType = dataType
        }
        if let brandOwner = try values.decodeIfPresent(String.self, forKey: .brandOwner) {
            self.brandOwner = brandOwner
        }
        if let gtinUpc = try values.decodeIfPresent(String.self, forKey: .gtinUpc) {
            self.gtinUpc = gtinUpc
        }
        if let ingredients = try values.decodeIfPresent(String.self, forKey: .ingredients) {
            self.ingredients = ingredients
        }
        if let publishedDate = try values.decodeIfPresent(String.self, forKey: .publishedDate) {
            self.publishedDate = publishedDate
        }
        if let score = try values.decodeIfPresent(Float.self, forKey: .score) {
            self.score = score
        }

    }

}

I've double checked to make sure all the types are correct, so I suspect that the problem is with missing keys.

This is the error I'm getting: typeMismatch(Swift.Int, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "foods", intValue: nil), _JSONKey(stringValue: "Index 4", intValue: 4), CodingKeys(stringValue: "ndbNumber", intValue: nil)], debugDescription: "Expected to decode Int but found a string/data instead.", underlyingError: nil))

The JSON is very long, so I'm just posting a short portion of it:

  "foodSearchCriteria": {
        "dataType": [
            "Foundation",
            "Branded"
        ],
        "query": "cheddar",
        "generalSearchInput": "cheddar",
        "pageNumber": 1,
        "requireAllWords": false
    },
    "totalHits": 12839,
    "currentPage": 1,
    "totalPages": 257,
    "foods": [
        {
            "fdcId": 566851,
            "description": "CHEDDAR",
            "dataType": "Branded",
            "gtinUpc": "828280001766",
            "publishedDate": "2019-04-01",
            "brandOwner": "FISCALINI",
            "ingredients": "PASTEURIZED COW'S MILK, CHEESE CULTURES, ENZYMES (MICROBIAL RENNET), SALT.",
            "foodNutrients": [
                {
                    "nutrientId": 1079,
                    "nutrientName": "Fiber, total dietary",
                    "nutrientNumber": "291",
                    "unitName": "G",
                    "derivationCode": "LCCD",
                    "derivationDescription": "Calculated from a daily value percentage per serving size measure",
                    "value": 0
                },
                {
                    "nutrientId": 1087,
                    "nutrientName": "Calcium, Ca",
                    "nutrientNumber": "301",
                    "unitName": "MG",
                    "derivationCode": "LCCD",
                    "derivationDescription": "Calculated from a daily value percentage per serving size measure",
                    "value": 714
                },
                {
                    "nutrientId": 1089,
                    "nutrientName": "Iron, Fe",
                    "nutrientNumber": "303",
                    "unitName": "MG",
                    "derivationCode": "LCCD",
                    "derivationDescription": "Calculated from a daily value percentage per serving size measure",
                    "value": 0
                },
                {
                    "nutrientId": 1104,
                    "nutrientName": "Vitamin A, IU",
                    "nutrientNumber": "318",
                    "unitName": "IU",
                    "derivationCode": "LCCD",
                    "derivationDescription": "Calculated from a daily value percentage per serving size measure",
                    "value": 1071
                },
                {
                    "nutrientId": 1162,
                    "nutrientName": "Vitamin C, total ascorbic acid",
                    "nutrientNumber": "401",
                    "unitName": "MG",
                    "derivationCode": "LCCD",
                    "derivationDescription": "Calculated from a daily value percentage per serving size measure",
                    "value": 0
                },
                {
                    "nutrientId": 1003,
                    "nutrientName": "Protein",
                    "nutrientNumber": "203",
                    "unitName": "G",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 26.79
                },
                {
                    "nutrientId": 1004,
                    "nutrientName": "Total lipid (fat)",
                    "nutrientNumber": "204",
                    "unitName": "G",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 32.14
                },
                {
                    "nutrientId": 1005,
                    "nutrientName": "Carbohydrate, by difference",
                    "nutrientNumber": "205",
                    "unitName": "G",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 3.57
                },
                {
                    "nutrientId": 1008,
                    "nutrientName": "Energy",
                    "nutrientNumber": "208",
                    "unitName": "KCAL",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 400
                },
                {
                    "nutrientId": 1093,
                    "nutrientName": "Sodium, Na",
                    "nutrientNumber": "307",
                    "unitName": "MG",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 679
                },
                {
                    "nutrientId": 1253,
                    "nutrientName": "Cholesterol",
                    "nutrientNumber": "601",
                    "unitName": "MG",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 111
                },
                {
                    "nutrientId": 1257,
                    "nutrientName": "Fatty acids, total trans",
                    "nutrientNumber": "605",
                    "unitName": "G",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 1.25
                },
                {
                    "nutrientId": 1258,
                    "nutrientName": "Fatty acids, total saturated",
                    "nutrientNumber": "606",
                    "unitName": "G",
                    "derivationCode": "LCCS",
                    "derivationDescription": "Calculated from value per serving size measure",
                    "value": 21.43
                }
            ],
            "allHighlightFields": "",
            "score": 721.74396

And here is a link with the full JSON: https://api.nal.usda.gov/fdc/v1/foods/search?query=cheddar&dataType=Foundation,Branded&api_key=DEMO_KEY

I'd appreciate any help on solving this.

Please read the error message. It says clearly that the value for key ndbNumber in the 5th item of the array foods is not an Int. But there is an obvious copy&paste error: You have to decode fdcId for key .fdcId rather than for .ndbNumber — vadian
You're right, it was a stupid copy and paste error. Thanks for noticing it. It works now. — lxirsh
When you have json with sometimes missing arguments, instead of given it default values on your struct, you can simply declare them as optionals: public var description: String? — paulRick

Frankenstein Frankenstein · Accepted Answer · 2020-06-03T16:16:22

You have type-mismatch for the key foods and ndbNumber, use this struct to decode:

struct ReturnedFoods: Codable {
    let foodSearchCriteria: FoodSearchCriteria
    let totalHits, currentPage, totalPages: Int
    let foods: [Food]
}

struct FoodSearchCriteria: Codable {
    let dataType: [String]
    let query, generalSearchInput: String
    let pageNumber: Int
    let requireAllWords: Bool
}

struct Food: Codable {
    let fdcID: Int
    let description, dataType, gtinUpc, publishedDate: String
    let brandOwner, ingredients: String
    let foodNutrients: [FoodNutrient]
    let allHighlightFields: String
    let score: Double

    enum CodingKeys: String, CodingKey {
        case fdcID = "fdcId"
        case description, dataType, gtinUpc, publishedDate, brandOwner, ingredients, foodNutrients, allHighlightFields, score
    }
}

struct FoodNutrient: Codable {
    let nutrientID: Int
    let nutrientName, nutrientNumber, unitName: String
    let derivationCode: DerivationCode
    let derivationDescription: String
    let value: Double

    enum CodingKeys: String, CodingKey {
        case nutrientID = "nutrientId"
        case nutrientName, nutrientNumber, unitName, derivationCode, derivationDescription, value
    }
}

enum DerivationCode: String, Codable {
    case lccd = "LCCD"
    case lccs = "LCCS"
}

Trouble decoding JSON using the Swift Decoder protocol

1 Answers