swiftdata

What this skill does

# SwiftData

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

## Contents

- [Model Definition](#model-definition)
- [ModelContainer Setup](#modelcontainer-setup)
- [CloudKit Sync](#cloudkit-sync)
- [CRUD Operations](#crud-operations)
- [`@Query in SwiftUI`](#query-in-swiftui)
- [#Predicate](#predicate)
- [FetchDescriptor](#fetchdescriptor)
- [Schema Versioning and Migration](#schema-versioning-and-migration)
- [Concurrency (`@ModelActor`)](#concurrency-modelactor)
- [SwiftUI Integration](#swiftui-integration)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

## Model Definition

Apply `@Model` to a **class** (not struct). Generates `PersistentModel`, `Observable`, `Sendable`.

```swift
@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 and other compatible `Codable` value types, and relationships to `@Model` classes.

## ModelContainer Setup

```swift
// 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))
```

## CloudKit Sync

`ModelConfiguration(..., cloudKitDatabase:)` opts a SwiftData store into
automatic CloudKit sync, but app entitlements still gate sync.

For any SwiftData CloudKit setup or schema-review task, include a separate
**Capabilities** verdict before schema findings:

- **Capabilities**: Xcode target has the iCloud capability with CloudKit enabled
  and the intended container selected, plus Background Modes > Remote
  notifications. Without these entitlements, automatic sync is not fully
  configured even if `cloudKitDatabase` is set.
- **Schema compatibility**: no `@Attribute(.unique)` or `#Unique`;
  relationships are optional, have explicit inverses where needed, and avoid
  `.deny`; large `Data` uses `@Attribute(.externalStorage)`.
- **Scalar attributes**: do not make every scalar optional just for CloudKit.
  Keep required scalars nonoptional when initializers, defaults, or migrations
  provide valid values.
- **Schema rollout**: initialize the development schema only in nonproduction
  builds, verify it in CloudKit Dashboard, promote before release, and treat
  production changes as additive only.

## CRUD Operations

```swift
// 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

```swift
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

```swift
#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, conditional expressions, optional chaining and binding, nil coalescing, type casting. **Avoid**: loops, nested declarations, mutations, and arbitrary unsupported method calls.

## FetchDescriptor

```swift
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

```swift
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 transformatio