Swiftdata

# 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)
- [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 (composite, iOS 18+), 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))
```

## 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.isFavori