My problem is that I have two Models in my app – Car and Refuel. First one is to contain just a name of a car. User can have few cars. When a user refuels a tank and wants to log it, he selects a car from those previously saved and fills out the rest of the data. The issue that I’m having, is that my app crashes when I try to save a model (probably because I’m setting the relationship wrong).

First error I’m getting when selecting a Car from a picker is:
Picker: the selection "RefuelApp.Car" is invalid and does not have an associated tag, this will give undefined results.

The second one is when I’m closing the sheet (when the data should be saved):

//In Refuel.swift
@Relationship var car: Car
{
    @storageRestrictions(accesses: _$backingData, initializes: _car)
    init(initialValue) {
        _$backingData.setValue(forKey: .car, to: initialValue)
        _car = _SwiftDataNoType()
    }
    get {
        _$observationRegistrar.access(self, keyPath: .car)
        return self.getValue(forKey: .car)
    }
    set {
        _$observationRegistrar.withMutation(of: self, keyPath: .car) {
            self.setValue(forKey: .car, to: newValue) // <----- Thread 1: EXC_BREAKPOINT (code=1, subcode=0x1cc1641e0)
        }
    }
}

Here is some sample code:

RefuelEditView.swift

import SwiftUI
import SwiftData

struct RefuelEditView: View {
    @Environment(.dismiss) private var dismiss
    @Environment(.modelContext) private var modelContext
    
    @Query private var cars: [Car]
    @Bindable var refill: Refuel
    
    let isNew: Bool
    @State private var selectedCar: Car
    
    init(refill: Refuel, car: Car = Car(), isNew: Bool = false) {
        self.refill = refill
        self.isNew = isNew
        self.selectedCar = car
    }
    
    var body: some View {
        Form {
            Section(header: Text("Car")) {
                Picker(selection: $selectedCar, label: Text("Car")) {
                    Text("--None--").tag(nil as Car?)
                    ForEach(cars) { car in
                        Text(car.name).tag(car) // <----- Do not know if this is a correct way to populate a picker
                    }
                }
            }
            Section(header: Text("Refuel details")) {
                DatePicker(selection: .init(projectedValue: $refill.timestamp), label: { Text("Date") })
                HStack{
                    LabeledContent("Odometer") {
                        TextField("km", value: $refill.odo, format: .number.grouping(.automatic))
                            .keyboardType(.decimalPad)
                    }
                }
            }
       }
       .navigationTitle(isNew ? "New Refuel" : "Refuel")
        .toolbar {
            if isNew {
                ToolbarItem(placement: .confirmationAction) {
                    Button("Done") {
                        let item = refill
                        item.car = selectedCar // <----- I guess this is where I try to save it in a wrong way
                        modelContext.insert(item)
                        dismiss()
                    }
                }
                ToolbarItem(placement: .cancellationAction) {
                    Button("Cancel") {
                        modelContext.delete(refill)
                        dismiss()
                    }
                }
            }
        }
    }
}

Car.swift

import Foundation
import SwiftData

@Model
final class Car: Identifiable {
    let id = UUID()
    var name: String
    @Relationship(deleteRule: .cascade, inverse: Refuel.car) var refuels: [Refuel]?
    
    init() {
        self.name = ""
    }
    
    init(name: String) {
        self.name = name
    }
}

Refulel.swift

import Foundation
import SwiftData

@Model
final class Refuel: Identifiable {
    let id = UUID()
    @Relationship var car: Car
    var timestamp: Date
    private var odo_backend: Double = 0
    
    init(car: Car) {
        self.car = car
        self.odo_backend = 0
        self.timestamp = .now
    }
    
    init(car: Car, timestamp: Date, odo: Double) {
        self.car = car
        self.odo_backend = odo
        self.timestamp = timestamp
    }
}