SwiftData

Persist, query, and manage structured data in iOS 26+ apps using SwiftData with Swift 6.3.

Contents

• Model Definition
• ModelContainer Setup
• CRUD Operations
• @Query in SwiftUI
• #Predicate
• FetchDescriptor
• Schema Versioning and Migration
• Concurrency (@ModelActor)
• SwiftUI Integration
• Common Mistakes
• Review Checklist
• References

Model Definition

Apply

@Model

to a class (not struct). Generates

PersistentModel

,

Observable

,

Sendable

.

@Model
class Trip {
    var name: String
    var destination: String
    var startDate: Date
    var endDate: Date
    var isFavorite: Bool = false
    @Attribute(.externalStorage) var imageData: Data?
    @Relationship(deleteRule: .cascade, inverse: \LivingAccommodation.trip)
    var accommodation: LivingAccommodation?
    @Transient var isSelected: Bool = false  // Always provide default

    init(name: String, destination: String, startDate: Date, endDate: Date) {
        self.name = name; self.destination = destination
        self.startDate = startDate; self.endDate = endDate
    }
}

@Attribute options:

.externalStorage

,

.unique

,

.spotlight

,

.allowsCloudEncryption

,

.preserveValueOnDeletion

(iOS 18+),

.ephemeral

,

.transformable(by:)

. Rename:

@Attribute(originalName: "old_name")

.

@Relationship:

deleteRule:

.cascade

/

.nullify

(default)/

.deny

/

.noAction

. Specify

inverse:

for reliable behavior. Unidirectional (iOS 18+):

inverse: nil

.

#Unique (iOS 18+):

#Unique<Person>([\.firstName, \.lastName])

-- compound uniqueness.

Inheritance (iOS 26+):

@Model class BusinessTrip: Trip { var company: String }

.

Supported types:

Bool

,

Int

/

UInt

variants,

Float

,

Double

,

String

,

Date

,

Data

,

URL

,

UUID

,

Decimal

,

Array

,

Dictionary

,

Set

,

Codable

enums,

Codable

structs (composite, iOS 18+), relationships to

@Model

classes.

ModelContainer Setup

// Basic
let container = try ModelContainer(for: Trip.self, LivingAccommodation.self)

// Configured
let config = ModelConfiguration("Store", isStoredInMemoryOnly: false,
    groupContainer: .identifier("group.com.example.app"),
    cloudKitDatabase: .private("iCloud.com.example.app"))
let container = try ModelContainer(for: Trip.self, configurations: config)

// With migration plan
let container = try ModelContainer(for: SchemaV2.Trip.self,
    migrationPlan: TripMigrationPlan.self)

// In-memory (previews/tests)
let container = try ModelContainer(for: Trip.self,
    configurations: ModelConfiguration(isStoredInMemoryOnly: true))

CRUD Operations

// CREATE
let trip = Trip(name: "Summer", destination: "Paris", startDate: .now, endDate: .now + 86400*7)
modelContext.insert(trip)
try modelContext.save()  // or rely on autosave

// READ
let trips = try modelContext.fetch(FetchDescriptor<Trip>(
    predicate: #Predicate { $0.destination == "Paris" },
    sortBy: [SortDescriptor(\.startDate)]))

// UPDATE -- modify properties directly; autosave handles persistence
trip.destination = "Rome"

// DELETE
modelContext.delete(trip)
try modelContext.delete(model: Trip.self, where: #Predicate { $0.isFavorite == false })

// TRANSACTION (atomic)
try modelContext.transaction {
    modelContext.insert(trip); trip.isFavorite = true
}

@Query in SwiftUI

struct TripListView: View {
    @Query(filter: #Predicate<Trip> { $0.isFavorite == true },
           sort: \.startDate, order: .reverse)
    private var favorites: [Trip]

    var body: some View { List(favorites) { trip in Text(trip.name) } }
}

// Dynamic query via init
struct SearchView: View {
    @Query private var trips: [Trip]
    init(search: String) {
        _trips = Query(filter: #Predicate<Trip> { trip in
            search.isEmpty || trip.name.localizedStandardContains(search)
        }, sort: [SortDescriptor(\.name)])
    }
    var body: some View { List(trips) { trip in Text(trip.name) } }
}

// FetchDescriptor query
struct RecentView: View {
    static var desc: FetchDescriptor<Trip> {
        var d = FetchDescriptor<Trip>(sortBy: [SortDescriptor(\.startDate)])
        d.fetchLimit = 5; return d
    }
    @Query(RecentView.desc) private var recent: [Trip]
    var body: some View { List(recent) { trip in Text(trip.name) } }
}

#Predicate

#Predicate<Trip> { $0.destination.localizedStandardContains("paris") }  // String
#Predicate<Trip> { $0.startDate > Date.now }                            // Date
#Predicate<Trip> { $0.isFavorite && $0.destination != "Unknown" }       // Compound
#Predicate<Trip> { $0.accommodation?.name != nil }                      // Optional
#Predicate<Trip> { $0.tags.contains { $0.name == "adventure" } }        // Collection

Supported:

==

,

!=

,

<

,

<=

,

>

,

>=

,

&&

,

||

,

!

,

contains()

,

allSatisfy()

,

filter()

,

starts(with:)

,

localizedStandardContains()

,

caseInsensitiveCompare()

, arithmetic, ternary, optional chaining, nil coalescing, type casting. Not supported: flow control, nested declarations, arbitrary method calls.

FetchDescriptor

var d = FetchDescriptor<Trip>(predicate: ..., sortBy: [...])
d.fetchLimit = 20; d.fetchOffset = 0
d.includePendingChanges = true
d.propertiesToFetch = [\.name, \.startDate]
d.relationshipKeyPathsForPrefetching = [\.accommodation]
let trips = try modelContext.fetch(d)
let count = try modelContext.fetchCount(d)
let ids = try modelContext.fetchIdentifiers(d)
try modelContext.enumerate(d, batchSize: 1000) { trip in trip.isProcessed = true }

Schema Versioning and Migration

enum SchemaV1: VersionedSchema {
    static var versionIdentifier = Schema.Version(1, 0, 0)
    static var models: [any PersistentModel.Type] { [Trip.self] }
    @Model class Trip { var name: String; init(name: String) { self.name = name } }
}

enum SchemaV2: VersionedSchema {
    static var versionIdentifier = Schema.Version(2, 0, 0)
    static var models: [any PersistentModel.Type] { [Trip.self] }
    @Model class Trip {
        var name: String; var startDate: Date?  // New property
        init(name: String) { self.name = name }
    }
}

enum TripMigrationPlan: SchemaMigrationPlan {
    static var schemas: [any VersionedSchema.Type] { [SchemaV1.self, SchemaV2.self] }
    static var stages: [MigrationStage] { [migrateV1toV2] }
    static let migrateV1toV2 = MigrationStage.lightweight(
        fromVersion: SchemaV1.self, toVersion: SchemaV2.self)
}

// Custom migration for data transformation
static let migrateV2toV3 = MigrationStage.custom(
    fromVersion: SchemaV2.self, toVersion: SchemaV3.self,
    willMigrate: nil,
    didMigrate: { context in
        let trips = try context.fetch(FetchDescriptor<SchemaV3.Trip>())
        for trip in trips { trip.displayName = trip.name.capitalized }
        try context.save()
    })

Lightweight handles: adding optional/defaulted properties, renaming (

originalName

), removing properties, adding model types.

Concurrency (@ModelActor)

@ModelActor
actor DataHandler {
    func importTrips(_ records: [TripRecord]) throws {
        for r in records {
            modelContext.insert(Trip(name: r.name, destination: r.dest,
                                    startDate: r.start, endDate: r.end))
        }
        try modelContext.save()  // Always save explicitly in @ModelActor
    }

    func process(tripID: PersistentIdentifier) throws {
        guard let trip = self[tripID, as: Trip.self] else { return }
        trip.isProcessed = true; try modelContext.save()
    }
}

let handler = DataHandler(modelContainer: container)
try await handler.importTrips(records)

Rules:

ModelContainer

is

Sendable

.

ModelContext

is NOT -- use on its creating actor. Pass

PersistentIdentifier

(Sendable) across boundaries. Never pass

@Model

objects across actors.

SwiftUI Integration

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup { ContentView() }
            .modelContainer(for: [Trip.self, LivingAccommodation.self])
    }
}

struct DetailView: View {
    @Environment(\.modelContext) private var modelContext
    let trip: Trip
    var body: some View {
        Text(trip.name)
        Button("Delete") { modelContext.delete(trip) }
    }
}

#Preview {
    let config = ModelConfiguration(isStoredInMemoryOnly: true)
    let container = try! ModelContainer(for: Trip.self, configurations: config)
    container.mainContext.insert(Trip(name: "Preview", destination: "London",
        startDate: .now, endDate: .now + 86400))
    return TripListView().modelContainer(container)
}

Common Mistakes

1. @Model on struct -- Use class.

@Model

requires reference semantics.

2. @Transient without default -- Always provide default:

@Transient var x: Bool = false

.

3. Missing .modelContainer -- @Query returns empty without a container on the view hierarchy.

4. Passing model objects across actors:

// WRONG: await handler.process(trip: trip)
// CORRECT: await handler.process(tripID: trip.persistentModelID)

5. ModelContext on wrong actor:

// WRONG: Task.detached { context.fetch(...) }
// CORRECT: Use @ModelActor for background work

6. Unsupported #Predicate expressions:

// WRONG: #Predicate<Trip> { $0.name.uppercased() == "PARIS" }
// CORRECT: #Predicate<Trip> { $0.name.localizedStandardContains("paris") }

7. Flow control in #Predicate:

// WRONG: #Predicate<Trip> { for tag in $0.tags { ... } }
// CORRECT: #Predicate<Trip> { $0.tags.contains { $0.name == "x" } }

8. No save in @ModelActor -- Always call

try modelContext.save()

explicitly.

9. ObservableObject with @Model -- Never use

ObservableObject

/

@Published

.

@Model

generates

Observable

. Use

@Query

in views.

10. Non-optional relationship without default:

// WRONG: var accommodation: LivingAccommodation  // crashes on reconstitution
// CORRECT: var accommodation: LivingAccommodation?

11. Cascade without inverse -- Specify

inverse:

for reliable cascade delete behavior.

12. DispatchQueue for background data work:

// WRONG: DispatchQueue.global().async { ModelContext(container).fetch(...) }
// CORRECT: @ModelActor actor Handler { func fetch() throws { ... } }

Review Checklist

• Every

  @Model

  is a class with a designated initializer
• All

  @Transient

  properties have default values
• Relationships specify

  deleteRule

  and

  inverse

• .modelContainer

  attached at scene/root view level

• @Query

  used for reactive data display in SwiftUI

• #Predicate

  uses only supported operators
• Background work uses

  @ModelActor

• PersistentIdentifier

  used across actor boundaries
• Schema changes have

  VersionedSchema

  +

  SchemaMigrationPlan

• Large data uses

  @Attribute(.externalStorage)

• CloudKit models use optionals and avoid unique constraints
• Explicit

  save()

  in

  @ModelActor

  methods
• Previews use

  ModelConfiguration(isStoredInMemoryOnly: true)

• @Model

  classes accessed from SwiftUI views are on

  @MainActor

  via

  @ModelActor

  or MainActor isolation

References

• See references/swiftdata-advanced.md for custom data stores, history tracking, CloudKit, Core Data coexistence, composite attributes, model inheritance, undo/redo, and performance patterns.
• See references/swiftdata-queries.md for @Query variants, FetchDescriptor deep dive, sectioned queries, dynamic queries, and background fetch patterns.
• See references/core-data-coexistence.md for standalone Core Data patterns and Core Data to SwiftData migration strategies.