7. Relasi Antar Model

SwiftData mendukung relasi one-to-one, one-to-many, dan many-to-many dengan @Relationship.

One-to-Many

swift
@Model
final class Author {
    var name: String
    var biography: String

    // One Author → many Books
    @Relationship(deleteRule: .cascade, inverse: \Book.author)
    var books: [Book] = []

    init(name: String, biography: String = "") {
        self.name = name
        self.biography = biography
    }
}

@Model
final class Book {
    var title: String
    var publishedYear: Int

    // Many Books → one Author (optional karena buku bisa orphan)
    var author: Author?

    init(title: String, publishedYear: Int) {
        self.title = title
        self.publishedYear = publishedYear
    }
}

Delete Rules

Delete Rule	Perilaku
`.nullify` (default)	Set relasi ke nil saat parent dihapus
`.cascade`	Hapus semua child saat parent dihapus
`.deny`	Tolak penghapusan parent jika masih punya child
`.noAction`	Tidak ada aksi — developer bertanggung jawab

Many-to-Many

swift
@Model
final class Book {
    var title: String

    @Relationship(inverse: \Tag.books)
    var tags: [Tag] = []

    init(title: String) {
        self.title = title
    }
}

@Model
final class Tag {
    var name: String
    var color: String

    @Relationship(inverse: \Book.tags)
    var books: [Book] = []

    init(name: String, color: String = "blue") {
        self.name = name
        self.color = color
    }
}

// Penggunaan
let book = Book(title: "Clean Code")
let tag = Tag(name: "Programming")
context.insert(book)
context.insert(tag)
book.tags.append(tag)  // otomatis update tag.books juga
try context.save()

Relasi Nested dan Lazy Loading

SwiftData menggunakan lazy loading untuk relasi — data relasi hanya diambil dari disk saat property diakses pertama kali. Ini efisien untuk graph model yang besar.

swift
@Model
final class Department {
    var name: String

    @Relationship(deleteRule: .cascade, inverse: \Employee.department)
    var employees: [Employee] = []   // tidak di-load sampai diakses

    var headCount: Int { employees.count }   // akses ini trigger lazy load

    init(name: String) { self.name = name }
}