The following complex migration consistently crashes the app with the following error:
SwiftData/PersistentModel.swift:726: Fatal error: What kind of backing data is this? SwiftData._KKMDBackingData<SwiftDataMigration.ItemSchemaV1.ItemList>
My app relies on a complex migration that involves these optional 1 to n relationships. Theoretically I could not assign the relationships in the willMigrate block but afterwards I am not able to tell which list and items belonged together.
Steps to reproduce:
Run project
Change typealias CurrentSchema to ItemSchemaV2 instead of ItemSchemaV1.
Run project again -> App crashes
My setup:
Xcode Version 16.2 (16C5032a)
MacOS Sequoia 15.4
iPhone 12 with 18.3.2 (22D82)
Am I doing something wrong or did I stumble upon a bug? I have a demo Xcode project ready but I could not upload it here so I put the code below.
Thanks for your help
typealias CurrentSchema = ItemSchemaV1
typealias ItemList = CurrentSchema.ItemList
typealias Item = CurrentSchema.Item
@main
struct SwiftDataMigrationApp: App {
var sharedModelContainer: ModelContainer = {
do {
return try ModelContainer(for: ItemList.self, migrationPlan: MigrationPlan.self)
} catch {
fatalError("Could not create ModelContainer: \(error)")
}
}()
var body: some Scene {
WindowGroup {
ContentView()
}
.modelContainer(sharedModelContainer)
}
}
This is the migration plan
enum MigrationPlan: SchemaMigrationPlan {
static var schemas: [any VersionedSchema.Type] {
[ItemSchemaV1.self, ItemSchemaV2.self]
}
static var stages: [MigrationStage] = [
MigrationStage.custom(fromVersion: ItemSchemaV1.self, toVersion: ItemSchemaV2.self, willMigrate: { context in
print("Started migration")
let oldlistItems = try context.fetch(FetchDescriptor<ItemSchemaV1.ItemList>())
for list in oldlistItems {
let items = list.items.map { ItemSchemaV2.Item(timestamp: $0.timestamp)}
let newList = ItemSchemaV2.ItemList(items: items, name: list.name, note: "This is a new property")
context.insert(newList)
context.delete(list)
}
try context.save() // Crash indicated here
print("Finished willMigrate")
}, didMigrate: { context in
print("Did migrate successfully")
})
]
}
The versioned schemas
enum ItemSchemaV1: VersionedSchema {
static var versionIdentifier = Schema.Version(1, 0, 0)
static var models: [any PersistentModel.Type] {
[Item.self]
}
@Model
final class Item {
var timestamp: Date
var list: ItemSchemaV1.ItemList?
init(timestamp: Date) {
self.timestamp = timestamp
}
}
@Model
final class ItemList {
@Relationship(deleteRule: .cascade, inverse: \ItemSchemaV1.Item.list)
var items: [Item]
var name: String
init(items: [Item], name: String) {
self.items = items
self.name = name
}
}
}
enum ItemSchemaV2: VersionedSchema {
static var versionIdentifier = Schema.Version(2, 0, 0)
static var models: [any PersistentModel.Type] {
[Item.self]
}
@Model
final class Item {
var timestamp: Date
var list: ItemSchemaV2.ItemList?
init(timestamp: Date) {
self.timestamp = timestamp
}
}
@Model
final class ItemList {
@Relationship(deleteRule: .cascade, inverse: \ItemSchemaV2.Item.list)
var items: [Item]
var name: String
var note: String
init(items: [Item], name: String, note: String = "") {
self.items = items
self.name = name
self.note = note
}
}
}
Last the ContentView:
struct ContentView: View {
@Query private var itemLists: [ItemList]
var body: some View {
NavigationSplitView {
List {
ForEach(itemLists) { list in
NavigationLink {
List(list.items) { item in
Text(item.timestamp.formatted(date: .abbreviated, time: .complete))
}
.navigationTitle(list.name)
} label: {
Text(list.name)
}
}
}
.navigationTitle("Crashing migration demo")
.onAppear {
if itemLists.isEmpty {
for index in 0..<10 {
let items = [Item(timestamp: Date.now)]
let listItem = ItemList(items: items, name: "List No. \(index)")
modelContext.insert(listItem)
}
try! modelContext.save()
}
}
} detail: {
Text("Select an item")
}
}
}
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