SwiftData looses attribute when restarting app

I'm trying to archive some data in my SwiftUI app, using SwiftData. I implemented a Archive Model:

import Foundation
import SwiftData


@Model
class ArchiveData {
    
    let archiveName: String
    let archiveDate: Date
    @Relationship(.cascade) var items: [Item]
    
    init(archiveName: String, archiveDate: Date, items: [Item]) {
        self.archiveName = archiveName
        self.archiveDate = archiveDate
        self.items = items
    }
    
}

Then, I'm adding the data to the context:

let newArchive = ArchiveData(archiveName: name, archiveDate: Date.now, items: items)
                    context.insert(newArchive)

I can then display the archived data in a separate view. But when I restart the app, the items array of ArchiveData is empty.

I couldn't find out, where I made a mistake. Can anyone help me?