SwiftData Persistence Patterns

Critical Constraints

• ❌ DO NOT use Core Data syntax (NSManagedObject, NSFetchRequest, NSPredicate) → ✅ Use @Model, @Query, #Predicate
• ❌ DO NOT use @FetchRequest → ✅ Use @Query
• ❌ DO NOT use string-based predicates → ✅ Use #Predicate<ModelType> macro with Swift expressions
• ❌ DO NOT create NSPersistentContainer → ✅ Use ModelContainer and ModelContext
• ❌ DO NOT forget @Model on subclasses → ✅ Both base class AND subclasses need @Model
• ❌ DO NOT use @Model on structs → ✅ @Model is for classes only

Basic Model Definition

import SwiftData

@Model
class Trip {
    @Attribute(.preserveValueOnDeletion)
    var name: String
    var destination: String
    var startDate: Date
    var endDate: Date

    @Relationship(deleteRule: .cascade, inverse: \Accommodation.trip)
    var accommodation: Accommodation?

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

Class Inheritance

Both base and subclass need @Model. Subclass adds specialized properties.

@Model
class BusinessTrip: Trip {
    var purpose: String
    var expenseCode: String
    var perDiemRate: Double

    @Relationship(deleteRule: .cascade, inverse: \BusinessMeal.trip)
    var businessMeals: [BusinessMeal] = []

    init(name: String, destination: String, startDate: Date, endDate: Date,
         purpose: String, expenseCode: String, perDiemRate: Double) {
        self.purpose = purpose
        self.expenseCode = expenseCode
        self.perDiemRate = perDiemRate
        super.init(name: name, destination: destination, startDate: startDate, endDate: endDate)
    }
}

@Model
class PersonalTrip: Trip {
    enum Reason: String, CaseIterable, Codable, Identifiable {
        case family, vacation, wellness, other
        var id: Self { self }
    }
    var reason: Reason
    var notes: String?

    init(name: String, destination: String, startDate: Date, endDate: Date,
         reason: Reason, notes: String? = nil) {
        self.reason = reason
        self.notes = notes
        super.init(name: name, destination: destination, startDate: startDate, endDate: endDate)
    }
}

Querying with Inheritance

// All trips (base + all subclasses)
@Query(sort: \Trip.startDate) var allTrips: [Trip]

// Filter by subclass type
let businessOnly = #Predicate<Trip> { $0 is BusinessTrip }
@Query(filter: businessOnly) var businessTrips: [Trip]

// Filter by subclass property
let vacations = #Predicate<Trip> {
    if let personal = $0 as? PersonalTrip {
        return personal.reason == .vacation
    }
    return false
}

ModelContainer Setup

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

// Custom configuration
let config = ModelConfiguration("MyStore", isStoredInMemoryOnly: false)
let container = try ModelContainer(for: Trip.self, configurations: config)

CRUD Operations

// Create
let trip = Trip(name: "Paris", destination: "France", startDate: .now, endDate: .now)
modelContext.insert(trip)

// Read
let descriptor = FetchDescriptor<Trip>(sortBy: [SortDescriptor(\.startDate)])
let trips = try modelContext.fetch(descriptor)

// Update — just modify properties, SwiftData auto-saves
trip.name = "Paris 2025"

// Delete
modelContext.delete(trip)

// Save explicitly
try modelContext.save()

Decision Tree: Inheritance vs Enum

Use inheritance when:
├── Clear IS-A relationship (BusinessTrip IS-A Trip)
├── Subclasses have significantly different properties
├── Need both deep queries (all trips) and shallow queries (business only)
└── Polymorphic relationships needed

Use enum/flag instead when:
├── Only a few shared properties
├── Distinction is simple (just a type tag)
├── Boolean flag suffices
└── Query strategy only focuses on specialized properties

Common Mistakes & Fixes

References

• SwiftData Documentation
• Adopting inheritance in SwiftData
• WWDC 2025: What's new in SwiftData