Core Data model version with new entity - ios

Question

I am unable to migrate my project to a new model version, and I have created the smallest project possible to highlight my issue, and hope that someone can show me what's wrong with my approach.
I have the project in GitHub: MigrateApp
I create a new project in Xcode, call it MigrateApp, choose SwiftUI and hook on the checkbox for Core Data. This should also work with UIKit, because I am not doing anything with the UI.
I immediately run the app, and click the plus button in the upper right a couple of times, to populate the Core Data with some items.
Then I stop the app from Xcode, and click the Core Data MigrateApp in Xcode. I click the Editor menu and choose Add Model Version. I choose the name MigrateApp v2. And I set the current model version to the new version, so that the green checkmark shows in MigrateApp v2.
The entities in MigrateApp v2 shows only one entity, called Item. I now want to add a new entity called Subitem, that have a one-to-one relation to Item. I add the Subitem entity, and adds two attributes, date with type Date and text with type String, both non-optional.
Then I add relationship sub to Item, with destination Subitem, and relationship item to Subitem with destination Item, and sets the inverse of both relationships.
Add a new file, choose Mapping Model type, set the source to MigrateApp.xcdatamodel and target data model to MigrateApp v2.xcdatamodel. Give it the name Modelv1Tov2.
I think that I have done everything correct up to now, so the next steps I am not quite sure I do correctly.
I open the Modelv1Tov2 mapping model file in Xcode, and sets the source of the Subitem entity mapping to Item in the Entity Mapping panel. This renames the Subitem to ItemToSubitem.
Clicking the ItemToItem, I see that the relationship mappings show sub. I click on sub, and sets the Key Path in Relationship Mapping to $source.sub, and mapping name to ItemToSubitem.
Likewise with ItemToSubitem, I click item relationship mappings, and set Key Path to $source.item and mapping name to ItemToItem.
The last thing I need to do, is ensure that Xcode migrate the database automatically, and don't try to infer the migration but use the migration mapping. Add the two lines to Persistence.swift in the init, after setting the container property:
container = NSPersistentContainer(name: "MigrateApp")
import CoreData
struct PersistenceController {
static let shared = PersistenceController()
static var preview: PersistenceController = {
let result = PersistenceController(inMemory: true)
let viewContext = result.container.viewContext
for _ in 0..<10 {
let newItem = Item(context: viewContext)
newItem.timestamp = Date()
}
do {
try viewContext.save()
} catch {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
let nsError = error as NSError
fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
}
return result
}()
let container: NSPersistentContainer
init(inMemory: Bool = false) {
container = NSPersistentContainer(name: "MigrateApp")
container.persistentStoreDescriptions.first?.shouldMigrateStoreAutomatically = true
container.persistentStoreDescriptions.first?.shouldInferMappingModelAutomatically = false
if inMemory {
container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
}
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
// Replace this implementation with code to handle the error appropriately.
// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
/*
Typical reasons for an error here include:
* The parent directory does not exist, cannot be created, or disallows writing.
* The persistent store is not accessible, due to permissions or data protection when the device is locked.
* The device is out of space.
* The store could not be migrated to the current model version.
Check the error message to determine what the actual problem was.
*/
fatalError("Unresolved error \(error), \(error.userInfo)")
}
})
container.viewContext.automaticallyMergesChangesFromParent = true
}
}
When I now runs the app again from Xcode, it crashes with the message: Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<NSManagedObject 0x600001d1fcf0> valueForUndefinedKey:]: the entity Item is not key value coding-compliant for the key "item".'
What have I done wrong here?

Answer 1

I have searched for answers to this problem, and I found that I do not need to set the shouldMigrateStoreAutomatically = true and shouldInferMappingModelAutomatically = false. If I let Core Data infer the migration, the migration will be successful. So I remade the steps above for a fresh project, and skipped the step where I added the two lines to Persistence.swift, and also skipped the step adding a mapping model, and the migration was successful. I would still want to know why I had the crash when using the mapping model, and will investigate that issue further.