axiom-realm-migration-ref

Realm to SwiftData Migration — Reference Guide

Purpose: Complete migration path from Realm to SwiftData Swift Version: Swift 5.9+ (Swift 6 with strict concurrency recommended) iOS Version: iOS 17+ (iOS 26+ recommended) Context: Realm Device Sync sunset Sept 30, 2025. This guide is essential for Realm users migrating before deadline.

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Critical Timeline

Realm Device Sync DEPRECATION DEADLINE = September 30, 2025

If your app uses Realm Sync:

• ⚠️ You MUST migrate by September 30, 2025
• ✅ SwiftData is the recommended replacement
• ⏰ Time remaining: Depends on current date, but migrations take 2-8 weeks for production apps

This guide provides everything needed for successful migration.

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Migration Strategy Overview

Phase 1 (Week 1-2): Preparation & Planning
├─ Audit current Realm usage
├─ Understand model relationships
├─ Plan data migration path
└─ Set up test environment

Phase 2 (Week 2-3): Development
├─ Create SwiftData models from Realm schemas
├─ Implement data migration logic
├─ Convert threading model to async/await
└─ Test with real data

Phase 3 (Week 3-4): Migration
├─ Migrate existing app users' data
├─ Run in parallel (Realm + SwiftData)
├─ Verify CloudKit sync works
└─ Monitor for issues

Phase 4 (Week 4+): Production
├─ Deploy update with parallel persistence
├─ Gradual cutover from Realm to SwiftData
├─ Deprecate Realm code
└─ Monitor CloudKit sync health

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Part 1: Pattern Equivalents

Model Definition Conversion

Realm → SwiftData: Basic Model

// REALM
class RealmTrack: Object {
    @Persisted(primaryKey: true) var id: String
    @Persisted var title: String
    @Persisted var artist: String
    @Persisted var duration: TimeInterval
    @Persisted var genre: String?
}

// SWIFTDATA
@Model
final class Track {
    @Attribute(.unique) var id: String
    var title: String
    var artist: String
    var duration: TimeInterval
    var genre: String?

    init(id: String, title: String, artist: String, duration: TimeInterval, genre: String? = nil) {
        self.id = id
        self.title = title
        self.artist = artist
        self.duration = duration
        self.genre = genre
    }
}

Key differences:

• Realm: @Persisted(primaryKey: true) → SwiftData: @Attribute(.unique)
• Realm: Implicit init → SwiftData: Explicit init required
• Realm: Object base class → SwiftData: @Model macro on final class

Realm → SwiftData: Relationships

// REALM: One-to-Many
class RealmAlbum: Object {
    @Persisted(primaryKey: true) var id: String
    @Persisted var title: String
    @Persisted var tracks: RealmSwiftCollection<RealmTrack>
}

// SWIFTDATA: One-to-Many
@Model
final class Album {
    @Attribute(.unique) var id: String
    var title: String

    @Relationship(deleteRule: .cascade, inverse: \Track.album)
    var tracks: [Track] = []
}

@Model
final class Track {
    @Attribute(.unique) var id: String
    var title: String
    var album: Album?  // Inverse automatically maintained
}

Key differences:

• Realm: Explicit RealmSwiftCollection type → SwiftData: Native [Track] array
• Realm: Manual relationship management → SwiftData: Inverse relationships automatic
• Realm: No delete rules → SwiftData: deleteRule: .cascade / .nullify / .deny

Realm → SwiftData: Indexes

// REALM
class RealmTrack: Object {
    @Persisted(primaryKey: true) var id: String
    @Persisted(indexed: true) var genre: String
    @Persisted(indexed: true) var releaseDate: Date
}

// SWIFTDATA
@Model
final class Track {
    @Attribute(.unique) var id: String
    @Attribute(.indexed) var genre: String = ""
    @Attribute(.indexed) var releaseDate: Date = Date()
}

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Part 2: Threading Model Conversion

Realm Threading → Swift Concurrency

Realm: Manual Thread Handling

class RealmDataManager {
    func fetchTracksOnBackground() {
        DispatchQueue.global().async {
            let realm = try! Realm()  // Must get Realm on each thread
            let tracks = realm.objects(RealmTrack.self)

            DispatchQueue.main.async {
                self.updateUI(tracks: Array(tracks))
            }
        }
    }

    func saveTrackOnBackground(_ track: RealmTrack) {
        DispatchQueue.global().async {
            let realm = try! Realm()
            try! realm.write {
                realm.add(track)
            }
        }
    }
}

Problems:

• Manual DispatchQueue threading error-prone
• Easy to access objects on wrong thread
• No compile-time guarantees

SwiftData: Actor-Based Concurrency

actor SwiftDataManager {
    let modelContainer: ModelContainer

    func fetchTracks() async -> [Track] {
        let context = ModelContext(modelContainer)
        let descriptor = FetchDescriptor<Track>()
        return (try? context.fetch(descriptor)) ?? []
    }

    func saveTrack(_ track: Track) async {
        let context = ModelContext(modelContainer)
        context.insert(track)
        try? context.save()
    }
}

// Usage (automatic thread handling)
@MainActor
class ViewController: UIViewController {
    @State private var tracks: [Track] = []
    private let manager: SwiftDataManager

    func loadTracks() async {
        tracks = await manager.fetchTracks()
    }
}

Advantages:

• No manual DispatchQueue
• Compile-time thread safety
• Automatic actor isolation
• Swift 6 strict concurrency compatible

Common Threading Patterns

Realm Pattern	SwiftData Pattern
DispatchQueue.global().async	async/await in background actor
realm.write { }	context.insert() + context.save()
Manual thread-local Realm instances	Shared ModelContainer + background ModelContext
Thread.isMainThread checks	@MainActor annotations

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Part 3: Schema Migration Strategies

Simple Schema Migration (Direct Conversion)

For apps with simple schemas (< 5 tables, < 10 fields), direct migration is straightforward:

actor SchemaImporter {
    let realmPath: String
    let modelContainer: ModelContainer

    func migrateFromRealm() async throws {
        // 1. Open Realm database
        let realmConfig = Realm.Configuration(fileURL: URL(fileURLWithPath: realmPath))
        let realm = try await Realm(configuration: realmConfig)

        // 2. Create SwiftData context
        let context = ModelContext(modelContainer)

        // 3. Migrate each model type
        try migrateAllTracks(from: realm, to: context)
        try migrateAllAlbums(from: realm, to: context)
        try migrateAllPlaylists(from: realm, to: context)

        // 4. Save all at once
        try context.save()

        print("Migration complete!")
    }

    private func migrateAllTracks(from realm: Realm, to context: ModelContext) throws {
        let realmTracks = realm.objects(RealmTrack.self)

        for realmTrack in realmTracks {
            let sdTrack = Track(
                id: realmTrack.id,
                title: realmTrack.title,
                artist: realmTrack.artist,
                duration: realmTrack.duration,
                genre: realmTrack.genre
            )
            context.insert(sdTrack)
        }
    }

    private func migrateAllAlbums(from realm: Realm, to context: ModelContext) throws {
        let realmAlbums = realm.objects(RealmAlbum.self)

        for realmAlbum in realmAlbums {
            let sdAlbum = Album(
                id: realmAlbum.id,
                title: realmAlbum.title
            )
            context.insert(sdAlbum)

            // Connect relationships after creating all records
            for realmTrack in realmAlbum.tracks {
                if let sdTrack = findTrack(id: realmTrack.id, in: context) {
                    sdAlbum.tracks.append(sdTrack)
                }
            }
        }
    }

    private func findTrack(id: String, in context: ModelContext) -> Track? {
        let descriptor = FetchDescriptor<Track>(
            predicate: #Predicate { $0.id == id }
        )
        return try? context.fetch(descriptor).first
    }
}

Complex Schema Migration (Transformation Layer)

For apps with complex schemas, many computed properties, or data transformations:

// Step 1: Define transformation layer
struct TrackDTO {
    let realmTrack: RealmTrack

    var id: String { realmTrack.id }
    var title: String { realmTrack.title }
    var cleanTitle: String { realmTrack.title.trimmingCharacters(in: .whitespaces) }
    var durationFormatted: String {
        let minutes = Int(realmTrack.duration) / 60
        let seconds = Int(realmTrack.duration) % 60
        return String(format: "%d:%02d", minutes, seconds)
    }
}

// Step 2: Migrate through transformation layer
actor ComplexMigrator {
    let modelContainer: ModelContainer

    func migrateWithTransformation(from realm: Realm) throws {
        let context = ModelContext(modelContainer)

        let realmTracks = realm.objects(RealmTrack.self)
        for realmTrack in realmTracks {
            let dto = TrackDTO(realmTrack: realmTrack)

            // Transform data during migration
            let sdTrack = Track(
                id: dto.id,
                title: dto.cleanTitle,  // Cleaned version
                artist: realmTrack.artist,
                duration: realmTrack.duration
            )
            context.insert(sdTrack)
        }

        try context.save()
    }
}

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Part 4: CloudKit Sync Transition

Realm Sync → SwiftData CloudKit

Realm Sync (now deprecated) provided automatic sync. SwiftData uses CloudKit directly:

// REALM SYNC: Automatic but deprecated
let config = Realm.Configuration(
    syncConfiguration: SyncConfiguration(user: app.currentUser!)
)

// SWIFTDATA: CloudKit (recommended replacement)
let schema = Schema([Track.self, Album.self])
let config = ModelConfiguration(
    schema: schema,
    cloudKitDatabase: .privat

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