Swiftdata Migration

SwiftData Migration Guide

Comprehensive guide for migrating from CoreData to SwiftData, managing schema versions, handling iCloud sync conflicts, and production-grade migration strategies.

Prerequisites

• iOS 17+ for SwiftData (iOS 26 recommended)
• Xcode 26+
• Familiarity with existing CoreData stack (if migrating)

CoreData to SwiftData Migration

Migration Strategy Overview

┌─────────────────────────────────────────────────────────────┐
│                    MIGRATION APPROACHES                      │
├─────────────────────────────────────────────────────────────┤
│                                                              │
│  1. COEXISTENCE (Recommended for large apps)                │
│     CoreData ←→ SwiftData running side-by-side              │
│     Gradual feature migration                                │
│                                                              │
│  2. COMPLETE MIGRATION (Smaller apps)                        │
│     CoreData → SwiftData one-time migration                  │
│     Requires downtime or careful orchestration               │
│                                                              │
│  3. FRESH START (New iCloud containers)                      │
│     New SwiftData store, export/import user data             │
│     Cleanest but requires user action                        │
│                                                              │
└─────────────────────────────────────────────────────────────┘

Step 1: Audit Your CoreData Model

Before migrating, analyze your existing model:

// CoreData Model Audit Checklist
/*
 ✓ List all entities and their attributes
 ✓ Document all relationships (to-one, to-many, many-to-many)
 ✓ Identify unique constraints
 ✓ Note transformable attributes and their types
 ✓ Check for derived attributes
 ✓ Review fetch request templates
 ✓ Audit NSManagedObject subclasses for custom logic
*/

// Example: Documenting CoreData entity
/*
 Entity: Note
 Attributes:
   - id: UUID (unique)
   - title: String (not optional)
   - content: String (optional)
   - createdAt: Date
   - isPinned: Bool (default: false)

 Relationships:
   - folder: Folder (to-one, optional, nullify)
   - tags: [Tag] (to-many, ordered)
*/

Step 2: Create Equivalent SwiftData Models

import SwiftData

// BEFORE: CoreData NSManagedObject
/*
@objc(Note)
class Note: NSManagedObject {
    @NSManaged var id: UUID
    @NSManaged var title: String
    @NSManaged var content: String?
    @NSManaged var createdAt: Date
    @NSManaged var isPinned: Bool
    @NSManaged var folder: Folder?
    @NSManaged var tags: NSOrderedSet?
}
*/

// AFTER: SwiftData @Model
@Model
class Note {
    // SwiftData generates its own persistent ID
    // Don't use @Attribute(.unique) for iCloud compatibility
    var legacyID: UUID?  // Keep for migration reference

    var title: String = ""
    var content: String = ""
    var createdAt: Date = Date()
    var isPinned: Bool = false

    // Relationships MUST be optional for iCloud
    var folder: Folder?
    var tags: [Tag]?  // Use array, not NSOrderedSet

    init(title: String, content: String = "") {
        self.title = title
        self.content = content
        self.createdAt = Date()
    }
}

Step 3: Data Migration Script

import CoreData
import SwiftData

actor DataMigrator {
    private let coreDataContainer: NSPersistentContainer
    private let swiftDataContainer: ModelContainer

    init(coreDataContainer: NSPersistentContainer, swiftDataContainer: ModelContainer) {
        self.coreDataContainer = coreDataContainer
        self.swiftDataContainer = swiftDataContainer
    }

    func migrateNotes(progressHandler: @escaping (Double) -> Void) async throws {
        let context = coreDataContainer.viewContext
        let swiftDataContext = ModelContext(swiftDataContainer)

        // Fetch all CoreData notes
        let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: "Note")
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "createdAt", ascending: true)]

        let coreDataNotes = try context.fetch(fetchRequest)
        let totalCount = Double(coreDataNotes.count)

        for (index, cdNote) in coreDataNotes.enumerated() {
            // Map CoreData object to SwiftData model
            let note = Note(
                title: cdNote.value(forKey: "title") as? String ?? "",
                content: cdNote.value(forKey: "content") as? String ?? ""
            )
            note.legacyID = cdNote.value(forKey: "id") as? UUID
            note.createdAt = cdNote.value(forKey: "createdAt") as? Date ?? Date()
            note.isPinned = cdNote.value(forKey: "isPinned") as? Bool ?? false

            swiftDataContext.insert(note)

            // Save in batches to avoid memory pressure
            if index % 100 == 0 {
                try swiftDataContext.save()
                progressHandler(Double(index) / totalCount)
            }
        }

        try swiftDataContext.save()
        progressHandler(1.0)
    }
}

Step 4: Coexistence Pattern

For gradual migration, run both stacks:

@main
struct MyApp: App {
    // CoreData stack for legacy features
    let coreDataController = CoreDataController.shared

    // SwiftData for new features
    let swiftDataContainer: ModelContainer

    init() {
        let schema = Schema([Note.self, Folder.self, Tag.self])
        let config = ModelConfiguration(
            schema: schema,
            cloudKitDatabase: .automatic
        )

        do {
            swiftDataContainer = try ModelContainer(for: schema, configurations: config)
        } catch {
            fatalError("SwiftData init failed: \(error)")
        }
    }

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(\.managedObjectContext, coreDataController.viewContext)
                .modelContainer(swiftDataContainer)
        }
    }
}

// Feature flag for gradual rollout
enum DataStoreFeature {
    static var useSwiftData: Bool {
        // Check if migration is complete
        UserDefaults.standard.bool(forKey: "swiftDataMigrationComplete")
    }
}

Schema Versioning

Versioned Schema Definition

// Version 1: Initial schema
enum NotesSchemaV1: VersionedSchema {
    static var versionIdentifier = Schema.Version(1, 0, 0)

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

    @Model
    class Note {
        var title: String = ""
        var content: String = ""
        var createdAt: Date = Date()

        init(title: String) {
            self.title = title
        }
    }
}

// Version 2: Add isPinned and folder relationship
enum NotesSchemaV2: VersionedSchema {
    static var versionIdentifier = Schema.Version(2, 0, 0)

    static var models: [any PersistentModel.Type] {
        [Note.self, Folder.self]
    }

    @Model
    class Note {
        var title: String = ""
        var content: String = ""
        var createdAt: Date = Date()
        var isPinned: Bool = false  // NEW
        var folder: Folder?         // NEW

        init(title: String) {
            self.title = title
        }
    }

    @Model
    class Folder {
        var name: String = ""
        @Relationship(inverse: \Note.folder)
        var notes: [Note]?

        init(name: String) {
            self.name = name
        }
    }
}

// Version 3: Add tags with many-to-many
enum NotesSchemaV3: VersionedSchema {
    static var versionIdentifier = Schema.Version(3, 0, 0)

    static var models: [any PersistentModel.Type] {
        [Note.self, Folder.self, Tag.self]
    }

    @Model
    class Note {
        var title: String = ""
        var content: String = ""
        var createdAt: Date = Date()
        var isPinned: Bool = false
        var folder: Folder?
        var tags: [Tag]?  // NEW

        init(title: String) {
            self.title = title
        }
    }

    @Model
    class Folder {
        var name: String = ""
        @Relationship(inverse: \Note.folder)
        var notes: [Note]?

        init(name: String) {
            self.name = name
        }
    }

    @Model
    class Tag {
        var name: String = ""
        @Relationship(inverse: \Note.tags)
        var notes: [Note]?

        init(name: String) {
            self.name = name
        }
    }
}

Migration Plan

enum NotesMigrationPlan: SchemaMigrationPlan {
    static var schemas: [any VersionedSchema.Type] {
        [NotesSchemaV1.self, NotesSchemaV2.self, NotesSchemaV3.self]
    }

    static var stages: [MigrationStage] {
        [migrateV1toV2, migrateV2toV3]
    }

    // Lightweight migration (no data transformation)
    static let migrateV1toV2 = MigrationStage.lightweight(
        fromVersion: NotesSchemaV1.self,
        toVersion: NotesSchemaV2.self
    )

    // Custom migration with data transformation
    static let migrateV2toV3 = MigrationStage.custom(
        fromVersion: NotesSchemaV2.self,
        toVersion: NotesSchemaV3.self,
        willMigrate: { context in
            // Pre-migration: Clean up orphaned data
            let descriptor = FetchDescriptor<NotesSchemaV2.Note>(
                predicate: #Predicate { $0.title.isEmpty }
            )
            let emptyNotes = try context.fetch(descriptor)
            for note in emptyNotes {
                context.delete(note)
            }
            try context.save()
        },
        didMigrate: { context in
            // Post-migration: Set default values or transform data
            let descriptor = FetchDescriptor<NotesSchemaV3.Note>()
            let notes = try context.fetch(descriptor)

            // Create default "Untagged" tag for notes without tags
            let untaggedTag = NotesSchemaV3.Tag(name: "Untagged")
            context.insert(untaggedTag)

            for note in notes where note.tags?.isEmpty ?? true {
                note.tags = [untaggedTag]
            }

            try context.save()
        }
    )
}

// Use migration plan in container
let container = try ModelContainer(
    for: NotesSchemaV3.Note.self, NotesSchemaV3.Folder.self, NotesSchemaV3.Tag.self,
    migrationPlan: NotesMigrationPlan.self
)

iCloud Sync Conflict Resolution

Understanding Sync Conflicts

┌─────────────────────────────────────────────────────────────┐
│                   iCLOUD SYNC TIMELINE                       │
├─────────────────────────────────────────────────────────────┤
│                                                              │
│  Device A: Edit note.title = "Hello"    ──────┐             │
│                                                │ CONFLICT!   │
│  Device B: Edit note.title = "World"    ──────┘             │
│                                                              │
│  CloudKit Resolution: LAST WRITER WINS                       │
│  (Based on modificationDate)                                 │
│                                                              │
└─────────────────────────────────────────────────────────────┘

Strategy 1: Last-Writer-Wins (Default)

@Model
class Note {
    var title: String = ""
    var content: String = ""

    // CloudKit uses this for conflict resolution
    var modificationDate: Date = Date()

    func update(title: String) {
        self.title = title
        self.modificationDate = Date()  // Update timestamp
    }
}

Strategy 2: Field-Level Merge

@Model
class Note {
    var title: String = ""
    var content: String = ""

    // Track individual field modifications
    var titleModifiedAt: Date = Date()
    var contentModifiedAt: Date = Date()

    func updateTitle(_ newTitle: String) {
        title = newTitle
        titleModifiedAt = Date()
    }

    func updateContent(_ newContent: String) {
        content = newContent
        contentModifiedAt = Date()
    }

    // Merge conflicting versions
    func merge(with other: Note) {
        if other.titleModifiedAt > self.titleModifiedAt {
            self.title = other.title
            self.titleModifiedAt = other.titleModifiedAt
        }
        if other.contentModifiedAt > self.contentModifiedAt {
            self.content = other.content
            self.contentModifiedAt = other.contentModifiedAt
        }
    }
}

Strategy 3: Operational Transformation for Text

@Model
class CollaborativeNote {
    var title: String = ""

    // Store operations instead of final state
    @Attribute(.externalStorage)
    var operationsData: Data?

    // Computed content from operations
    @Transient
    var content: String = ""

    struct Operation: Codable {
        enum Kind: Codable {
            case insert(position: Int, text: String)
            case delete(range: Range<Int>)
        }
        let kind: Kind
        let timestamp: Date
        let deviceID: String
    }

    var operations: [Operation] {
        get {
            guard let data = operationsData else { return [] }
            return (try? JSONDecoder().decode([Operation].self, from: data)) ?? []
        }
        set {
            operationsData = try? JSONEncoder().encode(newValue)
        }
    }

    func applyOperations() {
        var text = ""
        let sortedOps = operations.sorted { $0.timestamp < $1.timestamp }

        for op in sortedOps {
            switch op.kind {
            case .insert(let position, let insertText):
                let index = text.index(text.startIndex, offsetBy: min(position, text.count))
                text.insert(contentsOf: insertText, at: index)
            case .delete(let range):
                let start = text.index(text.startIndex, offsetBy: range.lowerBound)
                let end = text.index(text.startIndex, offsetBy: min(range.upperBound, text.count))
                text.removeSubrange(start..<end)
            }
        }

        content = text
    }
}

Strategy 4: Soft Deletes for Conflict Prevention

@Model
class Note {
    var title: String = ""
    var content: String = ""

    // Soft delete instead of hard delete
    var isDeleted: Bool = false
    var deletedAt: Date?
    var deletedBy: String?  // Device identifier

    func softDelete(deviceID: String) {
        isDeleted = true
        deletedAt = Date()
        deletedBy = deviceID
    }

    func restore() {
        isDeleted = false
        deletedAt = nil
        deletedBy = nil
    }
}

// Query only active notes
@Query(filter: #Predicate<Note> { !$0.isDeleted })
var activeNotes: [Note]

// Periodically purge soft-deleted items
func purgeDeletedNotes(olderThan days: Int, context: ModelContext) throws {
    let cutoff = Calendar.current.date(byAdding: .day, value: -days, to: Date())!

    try context.delete(model: Note.self, where: #Predicate { note in
        note.isDeleted && (note.deletedAt ?? Date()) < cutoff
    })
}

Monitoring Sync Status

import Combine

@Observable
class SyncMonitor {
    var syncState: SyncState = .idle
    var lastSyncDate: Date?
    var pendingChanges: Int = 0

    enum SyncState {
        case idle
        case syncing
        case error(String)
    }

    private var cancellables = Set<AnyCancellable>()

    init() {
        // Monitor CloudKit account status
        NotificationCenter.default.publisher(for: .CKAccountChanged)
            .sink { [weak self] _ in
                Task { await self?.checkAccountStatus() }
            }
            .store(in: &cancellables)

        // Monitor remote changes
        NotificationCenter.default.publisher(
            for: NSPersistentStoreRemoteChange
        )
        .sink { [weak self] notification in
            self?.handleRemoteChange(notification)
        }
        .store(in: &cancellables)
    }

    private func checkAccountStatus() async {
        // Check iCloud availability
    }

    private func handleRemoteChange(_ notification: Notification) {
        lastSyncDate = Date()
        syncState = .idle
    }
}

Production Migration Workflow

Pre-Migration Checklist

struct MigrationPreflight {

    /// Verify device has sufficient storage
    static func checkStorage() throws {
        let fileManager = FileManager.default
        let documentDirectory = fileManager.urls(for: .documentDirectory, in: .userDomainMask).first!

        let values = try documentDirectory.resourceValues(forKeys: [.volumeAvailableCapacityForImportantUsageKey])
        let availableBytes = values.volumeAvailableCapacityForImportantUsage ?? 0

        // Require at least 500MB for migration
        guard availableBytes > 500_000_000 else {
            throw MigrationError.insufficientStorage
        }
    }

    /// Backup current data before migration
    static func backupCoreDataStore() throws -> URL {
        let fileManager = FileManager.default
        let storeURL = CoreDataController.shared.persistentStoreURL

        let backupDirectory = fileManager.urls(for: .documentDirectory, in: .userDomainMask).first!
            .appendingPathComponent("Backups")

        try fileManager.createDirectory(at: backupDirectory, withIntermediateDirectories: true)

        let backupURL = backupDirectory
            .appendingPathComponent("CoreData_\(Date().ISO8601Format()).sqlite")

        try fileManager.copyItem(at: storeURL, to: backupURL)

        // Also backup -wal and -shm files if they exist
        let walURL = storeURL.appendingPathExtension("wal")
        let shmURL = storeURL.appendingPathExtension("shm")

        if fileManager.fileExists(atPath: walURL.path) {
            try fileManager.copyItem(at: walURL, to: backupURL.appendingPathExtension("wal"))
        }
        if fileManager.fileExists(atPath: shmURL.path) {
            try fileManager.copyItem(at: shmURL, to: backupURL.appendingPathExtension("shm"))
        }

        return backupURL
    }

    /// Verify data integrity before migration
    static func validateCoreDataIntegrity() async throws -> ValidationReport {
        let context = CoreDataController.shared.viewContext

        var report = ValidationReport()

        // Count all entities
        let notesFetch = NSFetchRequest<NSManagedObject>(entityName: "Note")
        report.notesCount = try context.count(for: notesFetch)

        let foldersFetch = NSFetchRequest<NSManagedObject>(entityName: "Folder")
        report.foldersCount = try context.count(for: foldersFetch)

        // Check for orphaned relationships
        let orphanedNotes = try context.fetch(notesFetch).filter { note in
            let folder = note.value(forKey: "folder") as? NSManagedObject
            return folder?.isDeleted ?? false
        }
        report.orphanedRelationships = orphanedNotes.count

        return report
    }
}

struct ValidationReport {
    var notesCount: Int = 0
    var foldersCount: Int = 0
    var orphanedRelationships: Int = 0

    var isValid: Bool {
        orphanedRelationships == 0
    }
}

enum MigrationError: LocalizedError {
    case insufficientStorage
    case backupFailed
    case validationFailed
    case migrationInterrupted

    var errorDescription: String? {
        switch self {
        case .insufficientStorage:
            return "Not enough storage space for migration. Please free up at least 500MB."
        case .backupFailed:
            return "Failed to create backup. Migration aborted."
        case .validationFailed:
            return "Data validation failed. Please contact support."
        case .migrationInterrupted:
            return "Migration was interrupted. Your data is safe in the backup."
        }
    }
}

Migration UI

struct MigrationView: View {
    @State private var migrationState: MigrationState = .notStarted
    @State private var progress: Double = 0
    @State private var error: MigrationError?

    enum MigrationState {
        case notStarted
        case preparingBackup
        case validating
        case migrating
        case verifying
        case completed
        case failed
    }

    var body: some View {
        VStack(spacing: 24) {
            Image(systemName: stateIcon)
                .font(.system(size: 60))
                .foregroundStyle(stateColor)

            Text(stateTitle)
                .font(.title2.bold())

            Text(stateDescription)
                .foregroundStyle(.secondary)
                .multilineTextAlignment(.center)

            if migrationState == .migrating {
                ProgressView(value: progress)
                    .progressViewStyle(.linear)

                Text("\(Int(progress * 100))%")
                    .font(.caption)
                    .foregroundStyle(.secondary)
            }

            if case .notStarted = migrationState {
                Button("Start Migration") {
                    Task { await startMigration() }
                }
                .buttonStyle(.borderedProminent)
            }

            if case .completed = migrationState {
                Button("Continue to App") {
                    UserDefaults.standard.set(true, forKey: "swiftDataMigrationComplete")
                }
                .buttonStyle(.borderedProminent)
            }

            if let error {
                Text(error.localizedDescription)
                    .foregroundStyle(.red)
                    .font(.caption)
            }
        }
        .padding()
    }

    private func startMigration() async {
        do {
            // Step 1: Backup
            migrationState = .preparingBackup
            _ = try MigrationPreflight.backupCoreDataStore()

            // Step 2: Validate
            migrationState = .validating
            let report = try await MigrationPreflight.validateCoreDataIntegrity()
            guard report.isValid else {
                throw MigrationError.validationFailed
            }

            // Step 3: Migrate
            migrationState = .migrating
            // ... migration logic with progress updates

            // Step 4: Verify
            migrationState = .verifying
            // ... verification logic

            migrationState = .completed

        } catch let migrationError as MigrationError {
            error = migrationError
            migrationState = .failed
        } catch {
            self.error = .migrationInterrupted
            migrationState = .failed
        }
    }

    private var stateIcon: String {
        switch migrationState {
        case .notStarted: return "arrow.triangle.2.circlepath"
        case .preparingBackup: return "externaldrive.badge.timemachine"
        case .validating: return "checkmark.shield"
        case .migrating: return "arrow.right.arrow.left"
        case .verifying: return "magnifyingglass"
        case .completed: return "checkmark.circle.fill"
        case .failed: return "xmark.circle.fill"
        }
    }

    private var stateColor: Color {
        switch migrationState {
        case .completed: return .green
        case .failed: return .red
        default: return .blue
        }
    }

    private var stateTitle: String {
        switch migrationState {
        case .notStarted: return "Ready to Migrate"
        case .preparingBackup: return "Creating Backup..."
        case .validating: return "Validating Data..."
        case .migrating: return "Migrating..."
        case .verifying: return "Verifying..."
        case .completed: return "Migration Complete!"
        case .failed: return "Migration Failed"
        }
    }

    private var stateDescription: String {
        switch migrationState {
        case .notStarted:
            return "We'll upgrade your data to the new format. This may take a few minutes."
        case .preparingBackup:
            return "Creating a backup of your data..."
        case .validating:
            return "Checking data integrity..."
        case .migrating:
            return "Transferring your notes to the new format..."
        case .verifying:
            return "Making sure everything transferred correctly..."
        case .completed:
            return "Your data has been successfully migrated!"
        case .failed:
            return "Something went wrong. Your original data is safe."
        }
    }
}

Best Practices Summary

DO

// ✓ Version your schemas from day one
enum MySchemaV1: VersionedSchema { ... }

// ✓ Use lightweight migrations when possible
MigrationStage.lightweight(fromVersion: V1.self, toVersion: V2.self)

// ✓ Design models for iCloud from the start
var folder: Folder?  // Optional relationships

// ✓ Keep legacy IDs for reference during migration
var legacyID: UUID?

// ✓ Create backups before migration
try MigrationPreflight.backupCoreDataStore()

// ✓ Use soft deletes for iCloud sync
var isDeleted: Bool = false

DON'T

// ✗ Don't use unique constraints with iCloud
@Attribute(.unique) var id: String  // NOT iCloud compatible

// ✗ Don't delete properties after shipping
// Instead, keep them but stop using them

// ✗ Don't change property types
var count: Int  // Can't change to String later

// ✗ Don't force-unwrap migrated data
let title = note.title!  // Could be nil during migration

// ✗ Don't skip validation
try container.mainContext.save()  // Always validate first

Official Resources

• SwiftData Documentation
• Adopting SwiftData for a Core Data app
• WWDC24: Create a custom data store with SwiftData
• WWDC23: Migrate to SwiftData