I hope I can make myself clear. I have two CoreData entities User and SecretItem. One User can hold an NSSet of SecrteItems. I set the relationship to Many at the user with destination SecretItem and to One at SecretItem with destination User. I use a two column NavigationSplitView in my ContenView (macos app) performing the @FetchRequest only here, not in the Detail-View:

struct ContentView: View {

    @Environment(\.managedObjectContext) var moc

    @FetchRequest(sortDescriptors: [SortDescriptor(\.name)])
    private var users: FetchedResults<User>

    @State private var selectedUserID: User.ID?
    @State private var defaultUserID: User.ID?

    var body: some View {
        NavigationSplitView {
            SidebarView(selection: selection)

        } detail: {
            if selection.wrappedValue != nil {
                SecretDetailView(user: selectedUser)
            } else {
                Text("Please select a user")
            }
        }
    }

    private var selection: Binding<User.ID?> {
        Binding(get: { selectedUserID ?? defaultUserID }, set: { selectedUserID = $0 })
    }

    private var selectedUser: User {
        users.filter({ $0.id == selection.wrappedValue }).first!         // <-- passed user to the @ObservedOject user in Detail-View
    }
}

Everything works fine. The users are listed in the Sidebar-View. A selected user is passed to ObservedObject var user: User in the Detail-View showing the secretItem details. When I try to add another secretItem the app crashes pointing to the Swift app-file with Thread 1: EXC_BREAKPOINT (code=1, subcode=0x19d0b56b4) I don't know how to add a new secretItem to the NSSet of the selected User. Any guidance?

here is my Detail-View with the add-func:

struct SecretDetailView: View {

    @Environment(\.managedObjectContext) var moc
    @ObservedObject var user: User

    var secretItems: [SecretItem] {
        return user.secretItems
            .filter {
                searchText.isEmpty ? true : $0.userName.localizedCaseInsensitiveContains(searchText)
            }
            .sorted(using: sortOrder)
    }

    @State var searchText: String = ""
    @State private var selection = Set<SecretItem.ID>()

    @State var sortOrder: [KeyPathComparator<SecretItem>] = [
        .init(\.userName, order: SortOrder.forward)
    ]

    // ... View Body stuff
}

extension SecretDetailView {

    // only to have somthing to work with!
    private func addSecretItems() {
        let usernames = ["quingflack23", "norton", "k-flex@hotmail.com", "kia-lokie@gmx.net", "san-silver", "quarks123", "user01", "wonder woman"]
        let passwords = ["1Jhg&%5", "123password", "noFun$3", "/(zhgTrF", "QTZ121!", "Wonder123)(", "awesomePW"]

        let chosenName = usernames.randomElement()!
        let chosenPW = passwords.randomElement()!

        Task { await addSecretItem(name: chosenName, pw: chosenPW)}
    }

    private func addSecretItem(name: String, pw: String) async {

        await moc.perform {
            let secretItem          = SecretItem(context: moc)
            secretItem.id           = UUID()
            secretItem.userName     = "\(name)"
            secretItem.password     = "\(pw)"
            secretItem.user         = user      // <-- here an error is thrown. I'm not sure if this is right, the SecretItem has an @NSManaged public var user: User created  

            user.addToSecretItems(secretItem)       // <-- this also throws an error. I try to use the created func in the User class
        }
        try? moc.save()
    }
}

I found my mistake myself. The issue was to be find in the @NSManagedObjectClass User. Within the created swift file User+CoreDataProperties.swift the generated accsessors for SecretItem was defiend wrong. I changed it as follows and than it worked as wanted:

User+CoreDataProperties.swift:

// MARK: Generated accessors for secretItems
extension User {
    ...
    @objc(addSecretItems:)
    @NSManaged public func addToSecretItems(_ values: Set <SecretItem>)
    ...
}

So my code within SecretDetailView was ok to have a new secretItem added to the user entity:

private func addSecretItem(name: String, pw: String) async {

        await moc.perform {
            let secretItem          = SecretItem(context: moc)
            secretItem.id           = UUID()
            secretItem.userName     = "\(name)"
            secretItem.password     = "\(pw)"
            secretItem.user         = user   // <-- here I'm still not sure if I need this

            print(secretItem)
            print(user)
            user.addToSecretItems(secretItem)
        }
        try? moc.save()
    }

Usually, you shouldn't need secretItem.user = user , the provided function addToSecretItems should do this for you. But if you do, you wouldn't need to call addToSecretItems on user.

But you could try and test it out.