SwiftData document-based app broken

Has anyone gotten SwiftData to work with SwiftUI's DocumentGroup? Even the (unmodified) Xcode Multiplatform document app template using SwiftData crashes when working with a saved document. I suspect that the DocumentGroup extension for SwiftData is not creating the context on the main thread, but only when re-opening saved documents. On macOS it crashes with the above mentioned error, but I have also seen this manifest as threading deadlocks (on the iPad and macOS).

With the all the work that was apparently put into SwiftData I'm incredulous that this would just flat out not work. I am hoping I am missing something, but (based on the unpopularity of UIManagedDocument) I fear that most people are simply not working with SwiftData in document-based apps or not using it with DocumentGroup (e.g. using it alongside Core Data).

I am hoping there is a work-around, as I have already invested a lot of time converting from Core Data.

In case anyone wants to dig further into this issue, I was able to demonstrate that the underlying NSManagedObjectContext is being created with "main queue concurrency" for new documents, but "private queue concurrency" when re-opening a document (I am guessing this is the issue).

I added the following debug code to my main view (which has @Environment(\.modelContext) private var modelContext declared):

.onAppear(perform: {
    if let moc = Mirror(reflecting: _modelContext.wrappedValue).descendant("_nsContext") as? NSManagedObjectContext {
        switch moc.concurrencyType {
        case .confinementConcurrencyType:
            print("Confinement Concurrency")
        case .privateQueueConcurrencyType:
            print("Private Queue Concurrency")
        case .mainQueueConcurrencyType:
            print("Main Queue Concurrency")
        }
        moc.perform {
            print("Context is on main thread??? -> \(Thread.current.isMainThread)")
        }
    }
})

[@Volker Runkel](https://developer.apple.com/forums/profile/Volker Runkel) - This isn't something I would recommend going to production with, and I doubt it's a complete solution (I am still seeing SQLite errors in the debug console), but in order to continue development I have come up with the following work-around (at least until I hear back from DTS):

@main
struct TestDocumentAppApp: App {
    var body: some Scene {
        DocumentGroup(editing: .itemDocument, migrationPlan: TestDocumentAppMigrationPlan.self) {
            IntermediarySceneView {
                ContentView()
            }
        }
    }
}

struct IntermediarySceneView<Content: View>: View {
    @Environment(\.modelContext) private var originalModelContext
    let content: Content

    var body: some View {
        content.environment(\.modelContext, originalModelContext.container.mainContext)
    }

    init(@ViewBuilder content: () -> Content) {
        self.content = content()
    }
}

Basically, all this is doing is overriding the modelContext variable by traversing back to the container and grabbing the mainContext (AFAIK that is what it should be). It seems to prevent crashes in my limited testing, but there is a good chance there are issues elsewhere (I'm not sure where else the incorrect context may still be in use), so YMMV.

FYI - I have been instructed by DTS to file a bug report for this issue. If anyone else is experiencing crashes or freezes when trying to implement a document-based app using SwiftData, please file a bug report so we can get the issue prioritized! If DTS are able to provide a work-around, I will post it here.

Response from DTS:

We believe this issue is a bug. Please file a bug report via Feedback Assistant (https://feedbackassistant.apple.com). For more information on Feedback Assistant, visit https://developer.apple.com/bug-reporting.

FWIW I filed one already. The more the merrier.

Updated macOS to 14.1 and Xcode to 15.0.1. Built my app and opened one of the existing documents that lead to above crash. Adding and deleting item now worked. Created a new document. All worked well after reopening. Seems the update fixed it for me. Can anyone confirm?

thx volker