SwiftData - Insert and delete

Hi, I started a new project selecting SwiftData as db by default.

I created my models as default.

@Model
class Trip {
    var id: UUID
    public private(set) var Name: String
    public private(set) var Members: [Member]
    public private(set) var Buys: [Buy]

    public func removeBuy(indexBuyToRemove: Int) {
        self.Buys.remove(at: indexBuyToRemove)
    }
}

@Model
class Member: Identifiable, Equatable {
    var id: UUID
    public private(set) var Name: String
}

@Model
class Buy: Identifiable, Equatable {
    var id: UUID
    public private(set) var Author: Member
    public private(set) var Ammount: Double
    public private(set) var Title: String
}

I initialize my trips as this in my ContentView

  @Query public var tripsList: [Trip]

And then pass this list in every other view.

I also have a ModelContext as default, passed like this in the @Main

@main
struct MainApp: App {
    var sharedModelContainer: ModelContainer = {
        let schema = Schema([
            Member.self, Buy.self, Trip.self
        ])
        let modelConfiguration = ModelConfiguration(schema: schema, isStoredInMemoryOnly: false)

        do {
            return try ModelContainer(for: schema, configurations: [modelConfiguration])
        } catch {
            fatalError("Could not create ModelContainer: \(error)")
        }
    }()

    var body: some Scene {
        WindowGroup {
            ContentView()
                .preferredColorScheme(.dark)
        }
        .modelContainer(sharedModelContainer)
    }
}

And defining in the other views like this

    @Environment(\.modelContext) var modelContext

Now. For the problem of consistency.

Whenever I add a new Trip I do this or delete (as .onDelete in the List)

public func addTrip(Trip: Trip) {
    modelContext.insert(Trip)
}

private func deleteItems(offsets: IndexSet) {
    withAnimation {
        for index in offsets {
            modelContext.delete(tripsList[index])
        }
    }
}

And everything works fine.

The problem starts when I try to delete a Buy element of the Trip.Buys: [Buy].

I do like this:

private func deleteItems(offsets: IndexSet) {
    withAnimation {
        for index in offsets {
            let buyToRemove = trip.Buys[index]
            trip.removeBuy(indexBuyToRemove: index)
            do {
                try modelContext.save()
            } catch {}
        }
    }
}

The delete is not consistent. What am I missing? What am I doing wrong?

Thank you for your help