Core Data Patterns — Expert Decisions

Expert decision frameworks for Core Data choices. Claude knows NSPersistentContainer and fetch requests — this skill provides judgment calls for when Core Data fits and architecture trade-offs.

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Decision Trees

Core Data vs Alternatives

What's your persistence need?
├─ Simple key-value storage
│  └─ UserDefaults or @AppStorage
│     Don't use Core Data for preferences
│
├─ Flat list of Codable objects
│  └─ Is query complexity needed?
│     ├─ NO → File-based (JSON/Plist) or SwiftData
│     └─ YES → Core Data or SQLite
│
├─ Complex relationships + queries
│  └─ How many objects?
│     ├─ < 10,000 → SwiftData (simpler) or Core Data
│     └─ > 10,000 → Core Data (more control)
│
├─ iCloud sync required
│  └─ NSPersistentCloudKitContainer
│     Built-in sync with Core Data
│
└─ Cross-platform (non-Apple)
   └─ SQLite directly or Realm
      Core Data is Apple-only

The trap: Using Core Data for simple lists. If you don't need relationships, queries, or undo, consider simpler options like SwiftData or file storage.

Context Architecture

How many contexts do you need?
├─ Simple app, UI-only operations
│  └─ viewContext only
│     Single context for reads and small writes
│
├─ Background imports/exports
│  └─ viewContext + newBackgroundContext()
│     Background for writes, viewContext for UI
│
├─ Complex with multiple writers
│  └─ Parent-child context hierarchy
│     Rarely needed — adds complexity
│
└─ Sync with server
   └─ Dedicated sync context
      performBackgroundTask for sync operations

Migration Strategy

What changed in your model?
├─ Added optional attribute
│  └─ Lightweight migration (automatic)
│
├─ Renamed attribute/entity
│  └─ Lightweight with mapping model hints
│     Set renaming identifier in model
│
├─ Changed attribute type
│  └─ Depends on conversion possibility
│     Int → String: lightweight
│     String → Date: may need custom
│
├─ Added required attribute (no default)
│  └─ Custom migration required
│     Or add default value to make lightweight
│
└─ Complex schema restructuring
   └─ Staged migration
      Multiple model versions, migrate step by step

Merge Policy Selection

What happens on save conflicts?
├─ UI context always wins
│  └─ NSMergeByPropertyObjectTrumpMergePolicy
│     Most common for view context
│
├─ Store (persisted) always wins
│  └─ NSMergeByPropertyStoreTrumpMergePolicy
│     For background sync contexts
│
├─ Need custom resolution
│  └─ Custom merge policy
│     Complex — avoid if possible
│
└─ Fail on conflict
   └─ NSErrorMergePolicy (default)
      Rarely want this

---

NEVER Do

Context Management

NEVER use viewContext for heavy operations:

// ❌ Blocks main thread during import
func importUsers(_ data: [UserData]) {
    let context = persistenceController.container.viewContext
    for item in data {
        let user = User(context: context)
        user.name = item.name
    }
    try? context.save()  // UI frozen!
}

// ✅ Use background context
func importUsers(_ data: [UserData]) async throws {
    try await persistenceController.container.performBackgroundTask { context in
        for item in data {
            let user = User(context: context)
            user.name = item.name
        }
        try context.save()
    }
}

NEVER pass NSManagedObjects between contexts:

// ❌ Object belongs to different context — crash or undefined behavior
let user = fetchUser(in: backgroundContext)
viewContext.delete(user)  // Wrong context!

// ✅ Re-fetch in target context using objectID
let user = fetchUser(in: backgroundContext)
let userInViewContext = viewContext.object(with: user.objectID) as! User
viewContext.delete(userInViewContext)

NEVER access managed objects off their context's queue:

// ❌ Thread violation — data corruption possible
let user = fetchUser(in: backgroundContext)
DispatchQueue.main.async {
    print(user.name)  // Accessing background object on main thread!
}

// ✅ Use context.perform for thread-safe access
backgroundContext.perform {
    let user = fetchUser(in: backgroundContext)
    let name = user.name
    DispatchQueue.main.async {
        print(name)  // Safe — using local copy
    }
}

Save Operations

NEVER ignore save errors:

// ❌ Silent data loss
try? context.save()

// ✅ Handle errors properly
do {
    try context.save()
} catch {
    context.rollback()
    Logger.coreData.error("Save failed: \(error)")
    throw error
}

NEVER save after every single change:

// ❌ Performance disaster — disk I/O per object
for item in largeDataset {
    let entity = Entity(context: context)
    entity.value = item
    try context.save()  // 10,000 saves!
}

// ✅ Batch changes, save once (or periodically)
for (index, item) in largeDataset.enumerated() {
    let entity = Entity(context: context)
    entity.value = item

    // Save every 1000 objects to manage memory
    if index % 1000 == 0 {
        try context.save()
        context.reset()  // Release memory
    }
}
try context.save()  // Final batch

NEVER call save on context with no changes:

// ❌ Unnecessary disk I/O
func periodicSave() {
    try? context.save()  // No-op but still has overhead
}

// ✅ Check for changes first
func saveIfNeeded() throws {
    guard context.hasChanges else { return }
    try context.save()
}

Fetch Optimization

NEVER fetch all objects when you need a count:

// ❌ Loads all objects into memory just to count
let users = try context.fetch(User.fetchRequest())
let count = users.count  // May fetch thousands!

// ✅ Use count fetch
let request = User.fetchRequest()
let count = try context.count(for: request)

NEVER fetch everything without limits:

// ❌ May load entire database
let request = User.fetchRequest()
let allUsers = try context.fetch(request)

// ✅ Set appropriate limits
let request = User.fetchRequest()
request.fetchLimit = 50
request.fetchBatchSize = 20  // Loads in batches

NEVER forget to prefetch relationships you'll access:

// ❌ N+1 problem — each post access triggers fault
let request = User.fetchRequest()
let users = try context.fetch(request)
for user in users {
    print(user.posts.count)  // Separate fetch per user!
}

// ✅ Prefetch relationships
let request = User.fetchRequest()
request.relationshipKeyPathsForPrefetching = ["posts"]
let users = try context.fetch(request)

Migration

NEVER assume lightweight migration will work:

// ❌ Crashes on incompatible changes
container.loadPersistentStores { _, error in
    if let error = error {
        fatalError("Failed: \(error)")  // User data lost!
    }
}

// ✅ Handle migration failure gracefully
container.loadPersistentStores { description, error in
    if let error = error as NSError? {
        if error.code == NSMigrationMissingSourceModelError {
            // Offer data reset or crash gracefully
            Self.resetStore()
        }
    }
}

---

Essential Patterns

Modern Persistence Controller

@MainActor
final class PersistenceController {
    static let shared = PersistenceController()
    static let preview = PersistenceController(inMemory: true)

    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "Model")

        if inMemory {
            container.persistentStoreDescriptions.first?.url = URL(fileURLWithPath: "/dev/null")
        }

        // Enable lightweight migration
        let description = container.persistentStoreDescriptions.first
        description?.shouldMigrateStoreAutomatically = true
        description?.shouldInferMappingModelAutomatically = true

        container.loadPersistentStores { _, error in
            if let error = error {
                // In production: log and handle gracefully
                fatalError("Core Data load failed: \(error)")
            }
        }

        // View context configuration
        container.viewContext.automaticallyMergesChangesFromParent = true
        container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
        container.viewContext.undoManager = nil  // Disable if not needed
    }

    func saveViewContext() {
        let context = container.viewContext
        guard context.hasChanges else { return }

        do {
            try context.save()
        } catch {
            Logger.coreData.error("View context save failed: \(error)")
        }
    }
}

Background Import Pattern

extension PersistenceController {
    func importData<T: Decodable>(
        _ items: [T],
        transform: @escaping (T, NSManagedObjectContext) -> Void
    ) async throws {
        try await container.performBackgroundTask { context in
            context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy

            for (index, item) in items.enumerated() {
                transform(item, context)

                // Batch save to manage memory
                if index > 0 && index % 500 == 0 {
                    try context.save()
                    context.reset()
                }
            }

            if context.hasChanges {
                try context.save()
            }
        }
    }
}

// Usage
try await persistenceController.importData(userDTOs) { dto, context in
    let user = User(context: context)
    user.id = dto.id
    user.name = dto.name
}

Efficient Fetch with @FetchRequest

struct UserListView: View {
    // Basic fetch — automatically updates on changes
    @FetchRequest(
        sortDescriptors: [SortDescriptor(\.name)],
        animation: .default
    )
    private var users: FetchedResults<User>

    var body: some View {
        List(users) { user in
            Text(user.name ?? "Unknown")
        }
    }
}

// Dynamic predicate fetch
struct FilteredUserList: View {
    @FetchRequest private var users: FetchedResults<User>

    init(searchText: String) {
        _users = FetchRequest(
            sortDescriptors: [SortDescriptor(\.name)],
            predicate: searchText.isEmpty ? nil : NSPredicate(
                format: "name CONTAINS[cd] %@", searchText
            ),
            animation: .default
        )
    }

    var body: some View {
        List(users) { user in
            Text(user.name ?? "")
        }
    }
}

---

Quick Reference

Core Data vs Alternatives

Need	Solution
Simple preferences	UserDefaults
Small Codable lists	JSON file or SwiftData
Complex queries + relationships	Core Data
iCloud sync	NSPersistentCloudKitContainer
Cross-platform	SQLite or Realm

Context Types

Context	Use For	Thread
viewContext	UI reads, small writes	Main
newBackgroundContext()	Heavy writes, imports	Background
performBackgroundTask	One-off background work	Background

Merge Policies

Policy	Winner	Use Case
ObjectTrump	In-memory changes	View context
StoreTrump	Persisted data	Sync context
ErrorMerge	Neither (fails)	Rarely wanted

Lightweight Migration Support

Change	Automatic?
Add optional attribute	✅ Yes
Add attribute with default	✅ Yes
Remove attribute	✅ Yes
Rename (with identifier)	✅ Yes
Change type (compatible)	✅ Maybe
Add required (no default)	❌ No
Change relationship type	❌ No

Red Flags

Smell	Problem	Fix
viewContext for imports	Main thread blocked	Use background context
NSManagedObject across contexts	Wrong thread access	Re-fetch via objectID
try? context.save()	Silent data loss	Handle errors
Save per object in loop	Disk I/O explosion	Batch saves
fetch() for count	Memory waste	context.count(for:)
No fetchLimit	Loads entire DB	Set reasonable limits
Missing prefetch	N+1 fetches	relationshipKeyPathsForPrefetching