swift / CoreData - Create "dummy" NSManagedObjecIDs for data models (to test things without the need of managed objects) - ios

Question

Say I have 2 NSManagedObjects in CoreData.
class House: NSManagedObject {}
class Location: NSManagedObject {}
I also have data model structs like this:
struct HouseModel {
var objectID: NSManagedObjectID
...
}
sruct LocationModel {
var objectID: NSManagedObjectID
...
}
For each loaded managedObject I basically use its attributes to initialize a new model struct to use for the UI and stuff (mainly collection views)
I have to have the NSManagedObjectID attribute in the structs in order to be able to make changes to the managedObject that struct belongs to. (I learned that I should use the mainViewContext only for reading while using something like persistentContainer.performBackgroundTask for writing. Thus, I need the NSManagedObjectID to load the objects on a background queue)
That's working but there is a problem with this approach:
I can't initialize one of these data models without a managed object. That's annoying when I want to create dummy data for UI testing or unit testing.
I know one solution: Create a Dummy managedObject with exactly one instance and use its objectID for stuff like that. But I don't really like this. Is there a better / more convenient way?
I mean, I would love to entirely remove the objectID attribute to keep CoreData separate from these model structs. But I don't see a way to do this. I need the connection.

Answer 1

For passing NSManagedObjects to a detail view for editing, it is often useful to do that on a new main queue managed object context, which simplifies your UI access and allows you to throw away the context if the user cancels changes.
But that's not what you asked.
Your problem is that you want to identify a managed object, but not use NSManagedObjectID. For this, you can use a URL property instead. NSManagedObjectID has a uriRepresentation() that returns a URL, and NSPersistentStoreCoordinator can convert a URL back into a managed object ID using managedObjectID(forURIRepresentation:). So you can store any old URL in the struct for testing purposes, and still be securely referring to managed objects in your app logic.