swiftdata-coredata-persistence

= Post . fetchRequest ( ) request . relationshipKeyPathsForPrefetching = [ ] // Don't prefetch let posts = try context . fetch ( request ) // Access triggers individual fault: let authorName = posts [ 0 ] . author . name // Only now loads User // ✅ Prefetch when you know you'll need it: request . relationshipKeyPathsForPrefetching = [ "author" ] Rule: Only prefetch relationships you'll access for ALL fetched objects. Pattern 4: NSFetchedResultsController (CoreData) For: Table/collection views with live updates. Why: Lazy loads, automatic UI updates, sectioning built-in. let request : NSFetchRequest < Post

= Post . fetchRequest ( ) request . sortDescriptors = [ NSSortDescriptor ( key : "createdAt" , ascending : false ) ] request . fetchBatchSize = 20 let frc = NSFetchedResultsController ( fetchRequest : request , managedObjectContext : context , sectionNameKeyPath : nil , cacheName : "PostsCache" ) frc . delegate = self try frc . performFetch ( ) // UITableView uses frc.fetchedObjects Rule: Use FRC for all CoreData-backed table/collection views. SwiftData Limitations (as of iOS 18) Feature CoreData SwiftData Workaround NSCompoundPredicate ✅ ❌ Multiple queries NSFetchedResultsController ✅ ❌ @Query in SwiftUI Abstract entities ✅ ❌ Use protocols Child contexts ✅ ❌ Use ModelContext Subclassing models ✅ ❌ Don't subclass Performance Faster Slower Wait for updates Common Mistakes Mistake Reality Fix "Migrations are automatic" Only lightweight migrations. Unique constraints, renames require complex migrations. Test with production data "I'll version later" Unversioned + iCloud = crash on first migration Version from day one "SwiftData = CoreData simplified" SwiftData has different rules (no subclassing, different delete rules) Learn SwiftData-specific constraints "externalStorage attribute fixes BLOBs" Still loaded into memory on fetch Store in file system "I can rename properties freely" With iCloud, renames blocked Add new property, keep old "Migration won't block UI" Large datasets block main thread Background initialization Migration Checklist Before shipping schema change: Defined versioned schemas (V1, V2) Written migration stage if complex (deduplication, renames) Tested with production data backup (1000+ records) Tested migration on background thread (no white screen) Verified iCloud sync works post-migration Checked new properties have defaults or are optional Confirmed unique constraints applied to clean data only Quick Reference SwiftData: // Basic model @Model class User { @Attribute ( . unique ) var email : String var name : String @Relationship ( deleteRule : . cascade ) var posts : [ Post ] } // Versioned schemas enum SchemaV1 : VersionedSchema { static var models : [ any PersistentModel . Type ] { [ User . self ] } @Model final class User { / ... / } } // Query (SwiftUI) @Query ( sort : \ . createdAt , order : . reverse ) var users : [ User ] CoreData: // Fetch with batching let request : NSFetchRequest < User

= User . fetchRequest ( ) request . fetchBatchSize = 20 request . predicate = NSPredicate ( format : "name CONTAINS[cd] %@" , searchText ) // Background context let bgContext = container . newBackgroundContext ( ) bgContext . perform { // Heavy work here try ? bgContext . save ( ) } Red Flags - STOP and Reconsider Shipped unversioned schema with iCloud → Ship versioning wrapper first Adding unique constraint to dirty data → Run deduplication migration Storing multi-MB files in database → Move to file system White screen on launch after update → Background migration Fetch taking >500ms → Add fetchBatchSize, check for BLOBs "I'll handle migration when users complain" → Test NOW with production data Real-World Impact Before: App crashes for 100% of users with iCloud enabled after first migration (unversioned→versioned trap). After: Ship v1.1 with versioning wrapper (no schema change), then v1.2 with actual changes. Zero crashes. Before: Feed loads in 4 seconds with 1000 posts + 2MB images each. After: Images moved to file system, fetchBatchSize = 20. Feed loads in 300ms, lazy-loads images.