Custom CodingKeys

[Rob Ryan]

Consider this model object:

struct Meal: BlackbirdModel, Identifiable {
    @BlackbirdColumn var id: String
    @BlackbirdColumn var name: String
    @BlackbirdColumn var thumbnailUrl: URL

    enum CodingKeys: String, CodingKey {
        case id = "idMeal"
        case name = "strMeal"
        case thumbnailUrl = "strMealThumb"
    }
}

For the sake of decoding a network resource with questionable naming conventions, I have introduced a CodingKeys to map their names to my own. Fine.

Blackbird creates a table with names of id, name, and thumbnailUrl. That seems good to me. But, when I try to save a record I get the following fatal error message:

2023-04-22 15:27:10.868259-0700 MyApp[49584:3615565] Blackbird/BlackbirdModel.swift:829: Fatal error: Table "Meal" definition defaults do not decode to model Meal: missingValue("idMeal")

Note, the database has the column name, but validateSchema(database:core:) is looking for strMeal (the name from CodingKeys). It looks like the logic it used for creating the column names (using the actual property names) is different than what validateSchema(database:core:) used. It strikes me that we should either use CodingKeys everywhere, or not at all. (I'd vote for the latter.)

Thoughts?

[marcoarment]

Well, I got it working. I kinda can't believe what it took. But I think it works!

Thanks.

[Rob Ryan]

@marcoarment, thank you for jumping that so quickly. That fixed the custom coding keys problem. (I was helping someone on this Stack Overflow question.)

FWIW, I tried a rendition with a custom decoder and things fell apart again, with this error:

Fatal error: … instances cannot be generated by simple decoding

Based upon the text of the error message, I gather that this is a known issue, so I won’t bother you further. But, if you want me to post a new issue with a MCVE, let me know. I am happy either way.

Thanks again.

[Rob Ryan]

Actually, I’ll just document it here so I don’t lose my train of thought. Bottom line, this works:

struct Meal: BlackbirdModel, Identifiable {
    @BlackbirdColumn var id: String
    @BlackbirdColumn var name: String
    @BlackbirdColumn var thumbnail: URL

    enum CodingKeys: String, BlackbirdCodingKey {
        case id = "idMeal"
        case name = "strMeal"
        case thumbnail = "strMealThumb"
    }

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

        self._id = try container.decode(BlackbirdColumn<String>.self, forKey: .id)
        self._name = try container.decode(BlackbirdColumn<String>.self, forKey: .name)
        self._thumbnail = try container.decode(BlackbirdColumn<URL>.self, forKey: .thumbnail)
    }
}

But the following results in the aforementioned “simple decoding” fatal error:

struct Meal: BlackbirdModel, Identifiable {
    @BlackbirdColumn var id: Int
    @BlackbirdColumn var name: String
    @BlackbirdColumn var thumbnail: URL

    enum CodingKeys: String, BlackbirdCodingKey {
        case id = "idMeal"
        case name = "strMeal"
        case thumbnail = "strMealThumb"
    }

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

        let string = try container.decode(String.self, forKey: .id)
        guard let value = Int(string) else {
            throw DecodingError.dataCorruptedError(forKey: .id, in: container, debugDescription: "Expected string representation of integer")
        }
        self._id = BlackbirdColumn<Int>(wrappedValue: value)
        self._name = try container.decode(BlackbirdColumn<String>.self, forKey: .name)
        self._thumbnail = try container.decode(BlackbirdColumn<URL>.self, forKey: .thumbnail)
    }
}

I haven't gone through it in any detail, but it feels like the problem is the overloading of the CodingKeys for both JSON and Blackbird.

[marcoarment]

Ah, I see. This is a trickier issue.

Blackbird needs a way to initialize empty instances of any BlackbirdModel struct, so it uses a Decoder trick (EmptyDecoder in the code) to automatically decode empty/zero default values for each data type.

In this case, the custom init method is asking for a String for .id, which it expects to convert to an Int manually.

But when Blackbird's EmptyDecoder is asked for a String, it returns an empty string (""), which fails the Int() conversion in this model and throws this error.

I can't think of a good way around this problem at the moment, other than not fighting it and simply requiring that any BlackbirdModel implementing a custom init(from decoder: Decoder) method must work when the decoder provides empty strings, zeros for numbers, etc. — or checks to see if decoder is an EmptyDecoder and provides a valid instance by some other method if so.

[marcoarment]

I've just improved the error message to reference the documentation about this feature, which now exists.

The special decoder used by Blackbird is now a public type (BlackbirdDefaultsDecoder) that can get special handling in client custom-decoding methods, so the above code can now be:

if decoder is BlackbirdDefaultsDecoder {
    self.id = 0
} else {
    let idStr = try container.decode(String.self, forKey: .id)
    guard let id = Int(idStr) else {
        throw DecodingError.dataCorruptedError(forKey: .id, in: container, debugDescription: "Expected numeric string")
    }
    self.id = id
}

self.name = try container.decode(String.self, forKey: .name)
self.thumbnail = try container.decode(URL.self, forKey: .thumbnail)

Note also that you can decode BlackbirdColumn values in a straightforward way using their wrapped type like I've done here — there's no need to decode the BlackbirdColumn<T> wrappers directly into the _-prefixed local variables.

[Rob Ryan]

Excellent.

FWIW, that decode logic with the _-prefixed variables was synthesized for me in the auto-complete process in Xcode 14.3. I didn't type that, but Xcode did. So I just accepted what it gave me, as it worked (until I tried changing a type in the manual unwrapping process). I should have questioned that auto-complete code…