Didn't even need to build or deploy an app with Beta 2 since Xcode 16.0 had already registered the new device. Install and launch Xcode 16.1 Beta 2 Open the Devices and Simulators window (Shift-Cmd-2) Let Xcode prepare the device for development and copy the shared cache symbols from the device Quit Xcode 16.1 Open Xcode 16.0 and build & deploy as usual

Your relationship should have a reference to the model(s) on both sides. Looks like you're missing a property on the Child model to refer to the Main model. I think you need the inverse attribute on your relationship on Main to identify the property I've only been able to get it to work if both sides of the relationship are optionals, even if the relationship is mandatory. So you need an optional trans array rather than an empty array. You'd create or fetch an instance of the Main model first, before creating the Child instances. Then create the Child instances, passing in a reference to their parent. You don't have to use an optional in the Child initializer to avoid accidentally leaving it nil. @Model final class Main { var name: String var limit: Double @Relationship(deleteRule: .cascade, inverse: \Child.parent) var trans: [Child]? init(name: String, limit: Double) { self.name = name self.limit = limit } } @Model final class Child { var trip: String var distance: Double var parent: Main? init(trip: String, distance: Double, parent: Main) { self.trip = trip self.distance = distance self.parent = parent } } let parent = Main(name: "", limit: 0) let child1 = Child(trip: "", distance: 0, parent: parent) let child2 = Child(trip: "", distance: 0, parent: parent) modelContext.insert(parent) modelContext.insert(child1) modelContext.insert(child2) modelContext.save() // the next time you fetch the parent, the children will be present in the `trans` array try modelContext.fetch(FetchDescriptor<Main>()) // or if you need them immediately in the code you're executing, you can assign them: parent.trans = [child1, child2]