I would recommend using a struct instead that mirrors the properties in the model, no more work really than working with duplicate properties and probably much easier to handle state changes etc.

Could it be that you create multiple instances of the ModelContainer?

You haven’t provided the details about CollectionItem which seems to be the class that causes the crash but I must say that it’s a very strange design you have where the superclass both has a children property of self and is being inherited. Do you really need such a complex solution?

Well a simplified version could be to only keep the current super class Item and use a Boolean (or an enum) property to define if it’s a collection or a link.

I would keep the ModelContainer as a stored property and create the ModelContext in each test or even better create both the container and context objects in each test. As for the issue with your initial code, the modelContext stored property is a reference to the modelContainer.mainContect but since the modelContainer object is local to the setUp method it will be released and the reference will no longer be valid when that method exits in your second solution the modelContext is a newly created object so it will remain in memory as long as the test is running.

Unrelated but times is a computed property so no need to mark it as Transient. You only need to do that for stored properties that shouldn't be persisted.

An array is a value type in itself even if the elements are reference types. So if you do var myArray = project.subcategories ?? [] then any change to myArray will not affect project.subcategories but if you make a change to an object in myArray (a Subproject object?) then that object in the project.subcategories array will also be updated. So I guess the short answer is that if you want to add or remove objects to the project you need to work with project.subcategories I can't really help you with the rest because I don't really understand what you are saying in the paragraph after the last code snippet.

MKMapPoint doesn’t conform to Codable so you can’t use it directly in your model. One solution is to store the coordinate properties in the model instead.

You don’t need to unwrap anything so just skip the if let self.project.subcategories?.removeAll { $0 == subcategory} calling a method on something that is nil in swift is perfectly fine

That’s a completely different issue and it’s not something you checked either in the code you have provided

My guess is that a view is trying to access an instance of the object after it has been removed.

The selection parameter for the picker should be a single item and not an array @State private var contractType: TypeOfContract = // some default type also the type of the value passed to .tag() must be the same type as the property above. So this issue is not so much about SwiftData but instead understanding how a picker works.

You should read the answers to your previous question carefully and also see this question on stackoverflow.com on using optionals with a picker

Here is a good Apple tutorial on handling errors in a view.

What do you mean with "more verbose way"? It seems to be the same command but one is wrapped in a generic function.