Crashes when accessing SwiftData model in View after it's been decoded from JSON, but it loads fine after the app reloads. What could be causing it?

Question

I have an app that request data from Reddit. When the view appears, in the onAppear modifier, it runs a function that fetches the data.

service.fetch(from: url, completion: { result in
    switch result {
    case .success(let listing):
        save(listing)
    case .failure:
        print("Failed to retrieve and decode data from Reddit")
    }
})

func save(_ listing: Listing) {
    modelContext.insert(listing)
    do {
        try modelContext.save()
    } catch {
        print("Error saving context")
        print(error.localizedDescription)
    }
}

func fetch(from url: URL, completion: @escaping (Result<Listing, Error>) -> Void) {
        
    let request = URLService.addUserAgentTo(url)
        
    session.dataTask(with: request, completionHandler: { data, urlResponse, error in
        // Check for error
        if let error = error {
            print("Error starting the session")
            print(error.localizedDescription)
            completion(.failure(error))
        } else {
            do {
                let data    = data ?? Data()
                let listing = try self.decoder.decode(Listing.self, from: data)
                completion(.success(listing))
            } catch {
                // Notify of failure to decode data
                print("Failed to decode Reddit data")
                print(error.localizedDescription)
                completion(.failure(error))
            }
        }
    }).resume()
}

In the view itself, all it does is print the post title names into a List.

List {
    ForEach(listings) { listing in
        ForEach(listing.data.children.sorted(by: {$0.data.timestamp < $1.data.timestamp})) { post in
            Text(post.data.title)
        }
    }
}

This is where the issue occurs. When it first tries to fetch data, it will crash and point to this code accessing $0.data. The message it provides you doesn't tell you much.

Thread 1: EXC_BREAKPOINT (code=1, subcode=0x1c3c5ca68)

Then it will also point to the Post model I have created and expand some code that seems to be internal to SwiftData and the @Model wrapper.

The weird thing is, if you just press the Start button in Xcode to rerun the app, it'll load all the data fine. So it clearly is decoding and saving it properly, but there seems to be some sort of disconnect between saving and viewing it. Although, I'm not sure why it would be an issue to begin with. If listings was empty it wouldn't try to access $0.data at all.

Does anyone have any ideas on things I can do to resolve this issue?

I don't know what to try. I've tried moving some code into DispatchQueue.main.async, but it didn't work. I also tried using Task.

Answer 1

I played around DispatchQueue some more, but I got it working after doing this.

do {
    try modelContext.save()
    modelContext.insert(listing)
    try modelContext.save()
} catch {
    print("There was an error saving the listing")
    print(error.localizedDescription)
}

I don't know why I would need to save the context prior to inserting the model. The save after the insert is required as well. Without it, it crashes with the same error.

I don't think this is a fix though. Just a workaround.