/ios

CoreData to SwiftData Migration

SwiftData replaces CoreData with a declarative, Swift-native persistence framework built on the @Model macro. Requires iOS 17 / macOS 14 minimum.

Concept Mapping

NSManagedObject to @Model

Before (CoreData)

// Requires .xcdatamodeld file with entity definition
// Codegen produces NSManagedObject subclass or you write manually:

class Item: NSManagedObject {
    @NSManaged var title: String
    @NSManaged var timestamp: Date
    @NSManaged var isComplete: Bool
    @NSManaged var tags: NSSet?  // To-many relationship
}

After (SwiftData)

import SwiftData

@Model
class Item {
    var title: String
    var timestamp: Date
    var isComplete: Bool
    var tags: [Tag]  // Direct Swift array, not NSSet

    init(title: String, timestamp: Date = .now, isComplete: Bool = false, tags: [Tag] = []) {
        self.title = title
        self.timestamp = timestamp
        self.isComplete = isComplete
        self.tags = tags
    }
}

Key differences:

• No .xcdatamodeld file needed. The schema is the Swift class itself.
• No @NSManaged. Properties are plain stored properties.
• Relationships use Swift arrays and optional types, not NSSet.
• You must provide an init -- @Model does not synthesize one.
• @Model automatically makes all stored properties persistent.

Excluding Properties from Persistence

@Model
class Item {
    var title: String
    var timestamp: Date

    // Not persisted
    @Transient var isSelected: Bool = false
}

Unique Constraints

@Model
class Item {
    #Unique<Item>([\.title, \.timestamp])

    var title: String
    var timestamp: Date
}

NSPersistentContainer to ModelContainer

Before (CoreData)

class PersistenceController {
    let container: NSPersistentContainer

    init() {
        container = NSPersistentContainer(name: "MyApp")
        container.loadPersistentStores { description, error in
            if let error {
                fatalError("Failed to load store: \(error)")
            }
        }
    }

    var viewContext: NSManagedObjectContext {
        container.viewContext
    }
}

After (SwiftData)

// Option 1: In SwiftUI (preferred)
@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(for: [Item.self, Tag.self])
    }
}

// Option 2: Manual configuration
let container = try ModelContainer(
    for: Item.self, Tag.self,
    configurations: ModelConfiguration(
        isStoredInMemoryOnly: false,
        allowsSave: true
    )
)

The .modelContainer modifier injects both the ModelContainer and a ModelContext into the SwiftUI environment.

NSFetchRequest to FetchDescriptor

Before (CoreData)

let request: NSFetchRequest<Item> = Item.fetchRequest()
request.predicate = NSPredicate(format: "isComplete == %@ AND title CONTAINS[cd] %@", NSNumber(value: false), searchText)
request.sortDescriptors = [NSSortDescriptor(keyPath: \Item.timestamp, ascending: false)]
request.fetchLimit = 20

let items = try viewContext.fetch(request)

After (SwiftData)

var descriptor = FetchDescriptor<Item>(
    predicate: #Predicate<Item> { item in
        !item.isComplete && item.title.localizedStandardContains(searchText)
    },
    sortBy: [SortDescriptor(\.timestamp, order: .reverse)]
)
descriptor.fetchLimit = 20

let items = try modelContext.fetch(descriptor)

Key differences:

• #Predicate is type-safe and checked at compile time. No format strings.
• SortDescriptor is Swift-native (not NSSortDescriptor).
• Fetch is called on ModelContext, not NSManagedObjectContext.

@FetchRequest to @Query

Before (CoreData)

struct ItemListView: View {
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: false)],
        predicate: NSPredicate(format: "isComplete == NO"),
        animation: .default
    )
    private var items: FetchedResults<Item>

    var body: some View {
        List(items) { item in
            ItemRow(item: item)
        }
    }
}

After (SwiftData)

struct ItemListView: View {
    @Query(
        filter: #Predicate<Item> { !$0.isComplete },
        sort: \.timestamp,
        order: .reverse
    )
    private var items: [Item]

    var body: some View {
        List(items) { item in
            ItemRow(item: item)
        }
    }
}

Key differences:

• @Query returns a plain [Item] array, not FetchedResults.
• Predicates use #Predicate macro.
• Sort is specified with keypaths and order directly.
• @Query automatically observes the ModelContext from the environment.

Dynamic Queries

If the predicate or sort needs to change at runtime, pass them via init:

struct ItemListView: View {
    @Query private var items: [Item]

    init(showCompleted: Bool) {
        let predicate: Predicate<Item>
        if showCompleted {
            predicate = #Predicate<Item> { _ in true }
        } else {
            predicate = #Predicate<Item> { !$0.isComplete }
        }
        _items = Query(filter: predicate, sort: \.timestamp, order: .reverse)
    }

    var body: some View {
        List(items) { item in
            ItemRow(item: item)
        }
    }
}

NSManagedObjectContext to ModelContext

Before (CoreData)

// Insert
let item = Item(context: viewContext)
item.title = "New Item"
item.timestamp = Date()

// Save
try viewContext.save()

// Delete
viewContext.delete(item)
try viewContext.save()

After (SwiftData)

// Insert
let item = Item(title: "New Item")
modelContext.insert(item)

// Save (automatic by default, or explicit)
try modelContext.save()

// Delete
modelContext.delete(item)
try modelContext.save()

Key differences:

• Objects are created with a normal Swift init, then inserted into the context.
• SwiftData auto-saves by default. Explicit save() is optional but recommended for critical operations.
• No context: parameter in the initializer.

Lightweight Migration (VersionedSchema + SchemaMigrationPlan)

SwiftData requires explicit migration stages when your schema changes between releases.

Define Versioned Schemas

enum ItemSchemaV1: VersionedSchema {
    static var versionIdentifier = Schema.Version(1, 0, 0)

    static var models: [any PersistentModel.Type] {
        [Item.self]
    }

    @Model
    class Item {
        var title: String
        var timestamp: Date

        init(title: String, timestamp: Date) {
            self.title = title
            self.timestamp = timestamp
        }
    }
}

enum ItemSchemaV2: VersionedSchema {
    static var versionIdentifier = Schema.Version(2, 0, 0)

    static var models: [any PersistentModel.Type] {
        [Item.self]
    }

    @Model
    class Item {
        var title: String
        var timestamp: Date
        var isComplete: Bool  // New property

        init(title: String, timestamp: Date, isComplete: Bool = false) {
            self.title = title
            self.timestamp = timestamp
            self.isComplete = isComplete
        }
    }
}

Define the Migration Plan

enum ItemMigrationPlan: SchemaMigrationPlan {
    static var schemas: [any VersionedSchema.Type] {
        [ItemSchemaV1.self, ItemSchemaV2.self]
    }

    static var stages: [MigrationStage] {
        [migrateV1toV2]
    }

    static let migrateV1toV2 = MigrationStage.lightweight(
        fromVersion: ItemSchemaV1.self,
        toVersion: ItemSchemaV2.self
    )
}

Use the Migration Plan

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
        .modelContainer(
            for: ItemSchemaV2.Item.self,
            migrationPlan: ItemMigrationPlan.self
        )
    }
}

For complex migrations that cannot be handled by lightweight migration, use .custom:

static let migrateV1toV2 = MigrationStage.custom(
    fromVersion: ItemSchemaV1.self,
    toVersion: ItemSchemaV2.self
) { context in
    // willMigrate: runs before schema changes
    // Transform data as needed with the OLD schema
} didMigrate: { context in
    // didMigrate: runs after schema changes
    // Transform data as needed with the NEW schema
    let items = try context.fetch(FetchDescriptor<ItemSchemaV2.Item>())
    for item in items {
        item.isComplete = false
    }
    try context.save()
}

Coexistence Strategy

You can run CoreData and SwiftData side by side during migration. This is the recommended approach for production apps with existing data.

Shared Persistent Store

Both frameworks can point to the same SQLite file:

let url = URL.applicationSupportDirectory.appending(path: "MyApp.store")

// CoreData
let description = NSPersistentStoreDescription()
description.url = url
let container = NSPersistentContainer(name: "MyApp")
container.persistentStoreDescriptions = [description]

// SwiftData
let config = ModelConfiguration(url: url)
let swiftDataContainer = try ModelContainer(for: Item.self, configurations: config)

Migration Strategy for Production Apps

1. Phase 1: Add SwiftData models alongside CoreData entities. New screens use SwiftData.
2. Phase 2: Migrate existing screens one at a time from CoreData to SwiftData.
3. Phase 3: Write a one-time data migration that copies remaining CoreData records to SwiftData.
4. Phase 4: Remove CoreData stack, .xcdatamodeld file, and NSManagedObject subclasses.

One-Time Data Migration

func migrateData(from coreDataContext: NSManagedObjectContext, to modelContext: ModelContext) throws {
    let request: NSFetchRequest<CDItem> = CDItem.fetchRequest()
    let coreDataItems = try coreDataContext.fetch(request)

    for cdItem in coreDataItems {
        let item = Item(
            title: cdItem.title ?? "",
            timestamp: cdItem.timestamp ?? .now,
            isComplete: cdItem.isComplete
        )
        modelContext.insert(item)
    }

    try modelContext.save()
}

When NOT to Migrate

Stay on CoreData if:

• Your minimum deployment target is below iOS 17 / macOS 14.
• You rely on CoreData features SwiftData does not yet support (e.g., abstract entities, derived attributes, fetched properties, complex multi-store configurations).
• You have a large, stable CoreData stack that is well-tested and not causing issues.
• You use CloudKit syncing with CoreData and have not verified SwiftData's CloudKit support meets your needs.
• Third-party libraries in your project depend on NSManagedObject subclasses.

Common Mistakes

// ❌ Forgetting to provide init for @Model class
@Model
class Item {
    var title: String
    var timestamp: Date
    // Compiler error: @Model requires explicit init
}

// ✅ Always provide init
@Model
class Item {
    var title: String
    var timestamp: Date

    init(title: String, timestamp: Date = .now) {
        self.title = title
        self.timestamp = timestamp
    }
}

// ❌ Using NSPredicate format strings with @Query
@Query(filter: NSPredicate(format: "isComplete == NO"))  // Wrong type
private var items: [Item]

// ✅ Use #Predicate macro
@Query(filter: #Predicate<Item> { !$0.isComplete })
private var items: [Item]

// ❌ Creating model objects with context parameter (CoreData habit)
let item = Item(context: modelContext)

// ✅ Create normally, then insert
let item = Item(title: "New")
modelContext.insert(item)

// ❌ Using NSSet for relationships
@Model
class Item {
    var tags: NSSet?  // Wrong, use Swift types
}

// ✅ Use Swift arrays
@Model
class Item {
    var tags: [Tag]
}

Checklist

• Minimum deployment target is iOS 17 / macOS 14
• All CoreData entities have equivalent @Model classes
• Relationships use Swift arrays/optionals, not NSSet
• All @Model classes have explicit init methods
• NSPredicate replaced with #Predicate macro
• NSSortDescriptor replaced with SortDescriptor
• @FetchRequest replaced with @Query
• NSManagedObjectContext usage replaced with ModelContext
• VersionedSchema and SchemaMigrationPlan set up for existing data
• One-time data migration tested (if coexisting with CoreData)
• CloudKit syncing verified (if applicable)
• .xcdatamodeld file removed after full migration