@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
    }
}

// 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))

// 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
}

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<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

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 }

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()
    })

@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)

@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)
}

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

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

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

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

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

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