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Swift Academy · Lesson

Relationships and Fetch Descriptors

Modelling one-to-many relationships and querying with FetchDescriptor.

Lesson 3 of 413 steps

Relationships and Fetch Descriptors is a free Swift Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

SwiftData Relationships

Declare relationships between @Model classes using Swift properties. SwiftData infers the relationship type.

@Model
class Author {
  var name: String
  @Relationship(deleteRule: .cascade) var books: [Book] = []
  init(name: String) { self.name = name }
}

@Relationship Attribute

Use @Relationship to specify delete rules and inverse relationships explicitly.

@Model
class Book {
  var title: String
  var author: Author?
  init(title: String) { self.title = title }
}

Delete Rules

Delete rules control what happens to related objects when the parent is deleted: .nullify, .cascade, .deny.

// .cascade: deleting Author deletes all Books
// .nullify: deleting Author sets book.author to nil
// .deny: prevents deletion if related objects exist
@Relationship(deleteRule: .cascade) var books: [Book] = []

One-to-Many Relationship

A one-to-many relationship is declared as an array property on the "one" side.

@Model class Department {
  var name: String
  @Relationship(deleteRule: .cascade) var employees: [Employee] = []
  init(name: String) { self.name = name }
}
@Model class Employee {
  var name: String
  var department: Department?
  init(name: String) { self.name = name }
}

Many-to-Many Relationships

Declare arrays on both sides for a many-to-many relationship.

@Model class Student {
  var name: String
  var courses: [Course] = []
  init(name: String) { self.name = name }
}
@Model class Course {
  var title: String
  var students: [Student] = []
  init(title: String) { self.title = title }
}

FetchDescriptor

FetchDescriptor replaces NSFetchRequest in SwiftData. Configure it with predicate, sort, and fetch limit.

var descriptor = FetchDescriptor<Book>(
  predicate: #Predicate { $0.title.contains("Swift") },
  sortBy: [SortDescriptor(\Book.title)]
)
descriptor.fetchLimit = 10
let books = try context.fetch(descriptor)

Sorting with SortDescriptor

Pass an array of SortDescriptor to FetchDescriptor for multi-column sorting.

let descriptor = FetchDescriptor<Item>(
  sortBy: [
    SortDescriptor(\Item.priority, order: .reverse),
    SortDescriptor(\Item.name)
  ]
)

Counting Without Fetching

Use context.fetchCount to count matching records without loading all objects into memory.

let descriptor = FetchDescriptor<Book>(
  predicate: #Predicate { $0.rating > 4 }
)
let count = try context.fetchCount(descriptor)
print("High-rated books: \(count)")

Prefetching Relationships

Set relationshipKeyPathsForPrefetching on a FetchDescriptor to eagerly load related objects.

var descriptor = FetchDescriptor<Author>()
descriptor.relationshipKeyPathsForPrefetching = [\Author.books]
let authors = try context.fetch(descriptor)

@Query with FetchDescriptor-like Options

The @Query macro supports the same sorting and predicate options as FetchDescriptor.

@Query(
  filter: #Predicate<Book> { $0.author?.name == "Apple" },
  sort: \Book.title
)
var books: [Book]

Batch Operations

SwiftData supports batch deletes via context.delete(model:where:) for efficient bulk operations.

try context.delete(model: Book.self, where: #Predicate { $0.rating < 2 })

Quick Check

Which SwiftData type replaces NSFetchRequest for fetching @Model objects with filters and sorting?

Answer it in the interactive lesson

Lesson Recap

Declare relationships with @Relationship(deleteRule:) on @Model classes. Use FetchDescriptor with #Predicate and SortDescriptor to query data. Use fetchCount for counting and batch delete for bulk removal.

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Frequently Asked Questions

Is the “Relationships and Fetch Descriptors” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Relationships and Fetch Descriptors”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “Relationships and Fetch Descriptors”?

Modelling one-to-many relationships and querying with FetchDescriptor. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Relationships and Fetch Descriptors” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Core Data Stack: NSPersistentContainer Setup
  2. SwiftData @Model and ModelContext
  3. Relationships and Fetch Descriptors
  4. Lightweight Migrations and CloudKit Sync
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