SwiftData Relationship Persistence

I'm still getting started with SwiftData and having trouble understanding how to persist a model with a relationship in a container. I've tried to distill the example below to be pretty simple.

Here are some example model definitions. I would like to be able to define a n Item. That Item can have SubItems related to it in a one-to-many fashion. Each SubItem is required to be attached to a parent Item.

final class Item {
    var name: String
    var subitems: [Item]?
    
    init(
        name: String,
        subitems: [SubItem]? = nil
    ) {
        self.name = name
    }
}

@Model
final class SubItem {
    var name: String
    
    init(name: String) {
        self.name = name
    }
}

In my app I am then defining a preview container with some pre-saved data already.

    Item(
        name: "item1",
        subitems: [
            SubItem(name: "subItemA"),
            SubItem(name: "subItemB")
        ]
    ),
    Item(
        name: "item2",
        subitems: [
            SubItem(name: "subItemC"),
            SubItem(name: "subItemD")
        ]
    )
]


@MainActor
let PreviewContainer: ModelContainer = {
    do {
        let schema = Schema([
            Item.self,
            SubItem.self,
        ])
        
        let container = try ModelContainer(
            for: schema, configurations: ModelConfiguration(isStoredInMemoryOnly: true)
        )
                
        for item in PreviewItems {
            container.mainContext.insert(item)
            
            for subItem in item.subitems! {
                container.mainContext.insert(subItem)
            }
        }
                
        return container
    } catch {
        fatalError("Failed to create container")
    }
}()

My app is then defined as follows...

struct SwiftDataTestApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(PreviewContainer)
    }
}

struct ContentView: View {
    @Environment(\.modelContext) private var modelContext
    @Query private var items: [Item]

    var body: some View {
        HStack {
            VStack {
                Text(items[0].name)
                Text(String(items[0].subitems!.count))
//                Text(items[0].subitems![0].name)
//                Text(items[0].subitems![1].name)
            }
            
            Spacer()
            
            VStack {
                Text(items[1].name)
                Text(String(items[1].subitems!.count))
//                Text(items[0].subitems![0].name)
//                Text(items[0].subitems![1].name)
            }
        }
        .padding(100)
    }
}

#Preview {
    ContentView()
        .modelContainer(PreviewContainer)
}

The preview loads without an issue, but if I uncomment the lines that access the SubItems it crashes. In the preview I can also see that each Item has 0 SubItems related to it. For some reason, the model container is not actually storing the `SubItem even though they are defined in PreviewItems.

Some things I've tried

• Explicitly adding the relationship
• Adding a Item property in SubItem to link back to it's parent
• In the PreviewContainer definition, manually insert the SubItems as well as the parent Items

Any help is appreciated, thanks