ios – Can’t add second child to parent in CoreData

As I struggled with developing my application I decided to learn some basics and created playground project using Core Data in SwiftUI, but I encountered problem I can’t resolve and even root cause. Let me begin with describing what I tried to achieve and what the problem is.

I created 3 models:

• ListEntity – have name and to-many relationship to ListItemEntity
• ListItemEntity – has just flag and 2 to-one relationship to ListEntity and NameEntity
• NameEntity – has just name and to-many relationship to ListItemEntity

For those models I created 3 basic views to show and add data. Views for showing and adding ListEntity (first for list of them and second with their details) works fine without any problem. The problem I have is with 3rd view which consists of searchable list of NameEntity (with add button, which just add new NameEntity of name Name <counts of names>). Each NameEntity row in list is clickable and by clicking it you should add (or delete) ListItemEntity to currently edited ListEntity with selected NameEntity. That works fine as long as you add only 1 item. When I try to add another I get error somewhere in context save method. The problem is, my save method is inside do { } catch { } block and it should print error to console, but instead it crashes whole application.

I created data in my persistent controller with more than just one ListItemEntity and it worked without problem:

let list = ListEntity(context: viewContext)
list.name = "List 1"
viewContext.forceSave()
        
var names = [NameEntity(context: viewContext), NameEntity(context: viewContext)]
for name in names {
    name.name = "Name \(names.firstIndex(of: name) ?? 0)"
}
viewContext.forceSave()
        
let items = [ListItemEntity(context: viewContext), ListItemEntity(context: viewContext)]
for item in items {
    item.flag = false
    item.list = list
    item.name = names.popLast()
}
viewContext.forceSave()

Here is error I’m getting:

error: Serious application error.  Exception was caught during Core Data change processing.  This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification.  -[NameEntity compare:]: unrecognized selector sent to instance 0x600002118ff0 with userInfo (null)

With call stack:

*** First throw call stack:

(

0   CoreFoundation                      0x0000000180491128 __exceptionPreprocess + 172

1   libobjc.A.dylib                     0x000000018008412c objc_exception_throw + 56

2   CoreFoundation                      0x00000001804a5f78 +[NSObject(NSObject) instanceMethodSignatureForSelector:] + 0

3   CoreFoundation                      0x0000000180495278 ___forwarding___ + 1280

4   CoreFoundation                      0x000000018049759c _CF_forwarding_prep_0 + 92

5   Foundation                          0x0000000180dde258 _NSCompareObject + 60

6   CoreData                            0x00000001863c8564 +[NSFetchedResultsController _insertIndexForObject:inArray:lowIdx:highIdx:sortDescriptors:] + 220

7   CoreData                            0x00000001863c82b4 -[NSFetchedResultsController _updateFetchedObjectsWithInsertChange:] + 832

8   CoreData                            0x00000001863c9e00 __82-[NSFetchedResultsController(PrivateMethods) _core_managedObjectContextDidChange:]_block_invoke + 2364

9   CoreData                            0x0000000186369e80 developerSubmittedBlockToNSManagedObjectContextPerform + 156

10  CoreData                            0x0000000186369d5c -[NSManagedObjectContext performBlockAndWait:] + 212

11  CoreData                            0x00000001863c94a8 -[NSFetchedResultsController _core_managedObjectContextDidChange:] + 96

12  CoreFoundation                      0x00000001803c1878 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 140

13  CoreFoundation                      0x00000001803c179c ___CFXRegistrationPost_block_invoke + 84

14  CoreFoundation                      0x00000001803c0c8c _CFXRegistrationPost + 404

15  CoreFoundation                      0x00000001803c0668 _CFXNotificationPost + 688

16  Foundation                          0x0000000180d84cb4 -[NSNotificationCenter postNotificationName:object:userInfo:] + 88

17  CoreData                            0x000000018635ce54 -[NSManagedObjectContext _postObjectsDidChangeNotificationWithUserInfo:] + 320

18  CoreData                            0x000000018636f640 -[NSManagedObjectContext _createAndPostChangeNotification:deletions:updates:refreshes:deferrals:wasMerge:] + 1244

19  CoreData                            0x000000018635eb9c -[NSManagedObjectContext _processRecentChanges:] + 2884

20  CoreData                            0x0000000186360cc0 -[NSManagedObjectContext save:] + 340

21  Playground                          0x00000001024b8da8 $sSo22NSManagedObjectContextC10PlaygroundE12saveIfNeededyyKF + 108

22  Playground                          0x00000001024b8eac $sSo22NSManagedObjectContextC10PlaygroundE9forceSaveyyF + 60

23  Playground                          0x00000001024c3d30 $s10Playground8NameTileV4bodyQrvgyycfU_ + 1220

24  SwiftUI                             0x00000001c5908800 OUTLINED_FUNCTION_11 + 620

25  SwiftUI                             0x00000001c58365c0 OUTLINED_FUNCTION_31 + 1824

26  SwiftUI                             0x00000001c51f742c OUTLINED_FUNCTION_21 + 32

27  SwiftUI                             0x00000001c4f04840 OUTLINED_FUNCTION_2 + 6392

28  SwiftUI                             0x00000001c4f0c7c4 OUTLINED_FUNCTION_2 + 39036

29  SwiftUI                             0x00000001c51f742c OUTLINED_FUNCTION_21 + 32

30  SwiftUI                             0x00000001c51f7448 OUTLINED_FUNCTION_21 + 60

31  SwiftUI                             0x00000001c51f742c OUTLINED_FUNCTION_21 + 32

32  SwiftUI                             0x00000001c58f6554 OUTLINED_FUNCTION_17 + 2340

33  SwiftUI                             0x00000001c58f6b18 OUTLINED_FUNCTION_17 + 3816

34  SwiftUI                             0x00000001c51e3f04 OUTLINED_FUNCTION_7 + 9760

35  SwiftUI                             0x00000001c51e9298 OUTLINED_FUNCTION_7 + 31156

36  UIKitCore                           0x0000000184a3583c -[UICollectionView _selectItemAtIndexPath:animated:scrollPosition:notifyDelegate:deselectPrevious:performCustomSelectionAction:] + 1176

37  UIKitCore                           0x0000000184a650b4 -[UICollectionView touchesEnded:withEvent:] + 452

38  UIKitCore                           0x000000018531a718 forwardTouchMethod + 264

39  UIKitCore                           0x000000018531a718 forwardTouchMethod + 264

40  UIKitCore                           0x000000018531a718 forwardTouchMethod + 264

41  UIKitCore                           0x0000000184e30458 _UIGestureEnvironmentUpdate + 5912

42  UIKitCore                           0x0000000184e2ea60 -[UIGestureEnvironment _deliverEvent:toGestureRecognizers:usingBlock:] + 288

43  UIKitCore                           0x0000000184e2e7d0 -[UIGestureEnvironment _updateForEvent:window:] + 156

44  UIKitCore                           0x0000000185329f00 -[UIWindow sendEvent:] + 3088

45  UIKitCore                           0x000000018530998c -[UIApplication sendEvent:] + 576

46  UIKitCore                           0x000000018538a5c0 __dispatchPreprocessedEventFromEventQueue + 1708

47  UIKitCore                           0x000000018538d474 __processEventQueue + 5524

48  UIKitCore                           0x0000000185385e38 __eventFetcherSourceCallback + 156

49  CoreFoundation                      0x00000001803f1f18 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 24

50  CoreFoundation                      0x00000001803f1e60 __CFRunLoopDoSource0 + 172

51  CoreFoundation                      0x00000001803f15d0 __CFRunLoopDoSources0 + 232

52  CoreFoundation                      0x00000001803ebcb8 __CFRunLoopRun + 768

53  CoreFoundation                      0x00000001803eb5a4 CFRunLoopRunSpecific + 572

54  GraphicsServices                    0x000000018e9fbae4 GSEventRunModal + 160

55  UIKitCore                           0x00000001852f02e4 -[UIApplication _run] + 868

56  UIKitCore                           0x00000001852f3f5c UIApplicationMain + 124

57  SwiftUI                             0x00000001c51fc1b0 OUTLINED_FUNCTION_70 + 500

58  SwiftUI                             0x00000001c51fc050 OUTLINED_FUNCTION_70 + 148

59  SwiftUI                             0x00000001c4f02fa4 OUTLINED_FUNCTION_2 + 92

60  Playground                          0x00000001024c9598 $s10Playground0A3AppV5$mainyyFZ + 40

61  Playground                          0x00000001024c9648 main + 12

62  dyld                                0x0000000102655544 start_sim + 20

63  ???                                 0x00000001028660e0 0x0 + 4337328352

64  ???                                 0xc719000000000000 0x0 + 14346498087965425664

)

Now here is my code, starting with data definition first:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

<model type="com.apple.IDECoreDataModeler.DataModel" documentVersion="1.0" lastSavedToolsVersion="22522" systemVersion="23C71" minimumToolsVersion="Automatic" sourceLanguage="Swift" usedWithSwiftData="YES" userDefinedModelVersionIdentifier="">

<entity name="ListEntity" representedClassName="ListEntity" syncable="YES">

<attribute name="name" optional="YES" attributeType="String"/>

<relationship name="items" optional="YES" toMany="YES" deletionRule="Cascade" destinationEntity="ListItemEntity" inverseName="list" inverseEntity="ListItemEntity"/>

</entity>

<entity name="ListItemEntity" representedClassName="ListItemEntity" syncable="YES">

<attribute name="flag" optional="YES" attributeType="Boolean" usesScalarValueType="YES"/>

<relationship name="list" optional="YES" maxCount="1" deletionRule="Nullify" destinationEntity="ListEntity" inverseName="items" inverseEntity="ListEntity"/>

<relationship name="name" optional="YES" maxCount="1" deletionRule="Nullify" destinationEntity="NameEntity" inverseName="items" inverseEntity="NameEntity"/>

</entity>

<entity name="NameEntity" representedClassName="NameEntity" syncable="YES">

<attribute name="name" optional="YES" attributeType="String"/>

<relationship name="items" optional="YES" toMany="YES" deletionRule="Nullify" destinationEntity="ListItemEntity" inverseName="name" inverseEntity="ListItemEntity"/>

</entity>

</model>

class DataProvider {
    static let shared = DataProvider()
    private let container: NSPersistentContainer

    var viewContext: NSManagedObjectContext {
        container.viewContext
    }

    private init() {
        container = NSPersistentContainer(name: "Data")
        container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
        container.loadPersistentStores { description, error in
            print(description.url?.path(percentEncoded: false) ?? "No URL")
            if let error {
                fatalError("Could not load persistent stores: \(error.localizedDescription)")
            }
        }
        container.viewContext.automaticallyMergesChangesFromParent = true
        container.viewContext.mergePolicy = NSMergePolicy.mergeByPropertyObjectTrump
    }
}

extension NSManagedObjectContext {
    func saveIfNeeded() throws {
        guard hasChanges else { return }
        try save()
    }

    func forceSave() {
        do {
            try saveIfNeeded()
        } catch {
            print(error.localizedDescription)
        }
    }
}

@objc(ListEntity)
public class ListEntity: NSManagedObject {}

public extension ListEntity {
    @nonobjc class func fetchRequest() -> NSFetchRequest<ListEntity> {
        NSFetchRequest<ListEntity>(entityName: "ListEntity")
    }

    @NSManaged var name: String?
    @NSManaged var items: NSSet?

    var wrappedName: String {
        name ?? "No name"
    }

    var wrappedItems: [ListItemEntity] {
        if let items = items as? Set<ListItemEntity> {
            return items.sorted(using: KeyPathComparator(\.wrappedName))
        }
        return []
    }
}

// MARK: Generated accessors for items

public extension ListEntity {
    @objc(addItemsObject:)
    @NSManaged func addToItems(_ value: ListItemEntity)

    @objc(removeItemsObject:)
    @NSManaged func removeFromItems(_ value: ListItemEntity)

    @objc(addItems:)
    @NSManaged func addToItems(_ values: NSSet)

    @objc(removeItems:)
    @NSManaged func removeFromItems(_ values: NSSet)
}

extension ListEntity: Identifiable {}

@objc(ListItemEntity)
public class ListItemEntity: NSManagedObject {}

public extension ListItemEntity {
    @nonobjc class func fetchRequest() -> NSFetchRequest<ListItemEntity> {
        NSFetchRequest<ListItemEntity>(entityName: "ListItemEntity")
    }

    @nonobjc class func fetchRequestFor(_ list: ListEntity, sortDescriptors: [NSSortDescriptor] = []) -> NSFetchRequest<ListItemEntity> {
        let fetchRequest = Self.fetchRequest()
        fetchRequest.predicate = NSPredicate(format: "list == %@", list)
        fetchRequest.sortDescriptors = sortDescriptors
        return fetchRequest
    }

    @nonobjc class func fetchRequest(with name: NameEntity, on list: ListEntity, sortDescriptors: [NSSortDescriptor] = []) -> NSFetchRequest<ListItemEntity> {
        let fetchRequest = Self.fetchRequest()
        fetchRequest.predicate = NSCompoundPredicate(andPredicateWithSubpredicates: [
            NSPredicate(format: "list == %@", list),
            NSPredicate(format: "name == %@", name),
        ])
        fetchRequest.sortDescriptors = sortDescriptors
        return fetchRequest
    }

    @NSManaged var flag: Bool
    @NSManaged var list: ListEntity?
    @NSManaged var name: NameEntity?

    var wrappedName: String {
        name?.wrappedName ?? "No name"
    }
}

extension ListItemEntity: Identifiable {}

@objc(NameEntity)
public class NameEntity: NSManagedObject {}

public extension NameEntity {
    @nonobjc class func fetchRequest() -> NSFetchRequest<NameEntity> {
        NSFetchRequest<NameEntity>(entityName: "NameEntity")
    }

    @nonobjc class func fetchRequest(containing name: String, fetchLimit: Int? = nil, sortDescriptors: [NSSortDescriptor] = []) -> NSFetchRequest<NameEntity> {
        let fetchRequest = Self.fetchRequest()
        if let fetchLimit {
            fetchRequest.fetchLimit = fetchLimit
        }
        let trimmedName = name.trimmingCharacters(in: .whitespacesAndNewlines)
        if !trimmedName.isEmpty {
            fetchRequest.predicate = NSPredicate(format: "name CONTAINS[cd] %@", trimmedName)
        }
        fetchRequest.sortDescriptors = sortDescriptors
        return fetchRequest
    }

    @NSManaged var name: String?
    @NSManaged var items: NSSet?

    var wrappedName: String {
        name ?? "No name"
    }

    var wrappedItems: [ListItemEntity] {
        if let items = items as? Set<ListItemEntity> {
            return items.sorted(using: KeyPathComparator(\.wrappedName))
        }
        return []
    }
}

// MARK: Generated accessors for items

public extension NameEntity {
    @objc(addItemsObject:)
    @NSManaged func addToItems(_ value: ListItemEntity)

    @objc(removeItemsObject:)
    @NSManaged func removeFromItems(_ value: ListItemEntity)

    @objc(addItems:)
    @NSManaged func addToItems(_ values: NSSet)

    @objc(removeItems:)
    @NSManaged func removeFromItems(_ values: NSSet)
}

extension NameEntity: Identifiable {}

Lastly my Views:

struct ListEntitiesView: View {
    @Environment(\.managedObjectContext)
    private var viewContext
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \ListEntity.name, ascending: true)],
        animation: .default
    )
    private var lists: FetchedResults<ListEntity>

    var body: some View {
        NavigationStack {
            List {
                ForEach(lists) { list in
                    NavigationLink(value: list) {
                        ListEntityTile(list)
                    }
                }
            }
            .navigationTitle("ListEntities")
            .navigationBarTitleDisplayMode(.inline)
            .toolbar {
                Button {
                    let list = ListEntity(context: viewContext)
                    list.name = "List \(lists.count + 1)"
                    viewContext.forceSave()
                } label: {
                    Label("Add List", systemImage: "plus")
                }
            }
            .navigationDestination(for: ListEntity.self) { list in
                ListEntityView(list)
            }
        }
    }
}

struct ListEntityTile: View {
    @ObservedObject
    private var list: ListEntity
    @FetchRequest
    private var items: FetchedResults<ListItemEntity>

    private var totalItems: Int {
        items.count
    }

    private var flaggedItems: Int {
        items.filter(\.flag).count
    }

    init(_ list: ListEntity) {
        _list = ObservedObject(wrappedValue: list)
        _items = FetchRequest(fetchRequest: ListItemEntity.fetchRequestFor(list))
    }

    var body: some View {
        HStack {
            Text(list.wrappedName)
            Spacer()
            Text("\(flaggedItems) / \(totalItems)")
        }
    }
}

struct ListEntityView: View {
    @Environment(\.managedObjectContext)
    private var viewContext
    @ObservedObject
    private var list: ListEntity
    @FetchRequest
    private var items: FetchedResults<ListItemEntity>

    init(_ list: ListEntity) {
        _list = ObservedObject(wrappedValue: list)
        _items = FetchRequest(fetchRequest: ListItemEntity.fetchRequestFor(list, sortDescriptors: [NSSortDescriptor(keyPath: \ListItemEntity.name, ascending: true)]))
    }

    var body: some View {
        List {
            ForEach(items) { item in
                ListItemTile(item)
            }
        }
        .navigationTitle(list.wrappedName)
        .navigationBarTitleDisplayMode(.inline)
        .toolbar {
            NavigationLink {
                AddListItemEntity(list)
            } label: {
                Label("Add Item", systemImage: "plus")
            }
        }
    }
}

struct ListItemTile: View {
    @Environment(\.managedObjectContext)
    private var viewContext
    @ObservedObject
    private var item: ListItemEntity

    init(_ item: ListItemEntity) {
        _item = ObservedObject(wrappedValue: item)
    }

    var body: some View {
        Button {
            item.flag.toggle()
            viewContext.forceSave()
        } label: {
            Label(item.wrappedName, systemImage: item.flag ? "checkmark.circle" : "circle")
        }
    }
}

struct AddListItemEntity: View {
    @Environment(\.managedObjectContext)
    private var viewContext
    @ObservedObject
    private var list: ListEntity
    @State
    private var searchName = ""
    @FetchRequest(sortDescriptors: [])
    private var names: FetchedResults<NameEntity>

    init(_ list: ListEntity) {
        _list = ObservedObject(wrappedValue: list)
    }

    var body: some View {
        List {
            TextField("Search name", text: $searchName.animation())
            NamesList(for: list, containing: searchName)
        }
        .toolbar {
            Button {
                let name = NameEntity(context: viewContext)
                name.name = "Name \(names.count + 1)"
                viewContext.forceSave()
            } label: {
                Label("Add Name", systemImage: "plus")
            }
        }
    }
}

struct NamesList: View {
    @ObservedObject
    private var list: ListEntity
    @FetchRequest
    private var names: FetchedResults<NameEntity>

    init(for list: ListEntity, containing name: String) {
        _list = ObservedObject(wrappedValue: list)
        _names = FetchRequest(
            fetchRequest: NameEntity.fetchRequest(containing: name, sortDescriptors: [NSSortDescriptor(keyPath: \NameEntity.name, ascending: true)]),
            animation: .default
        )
    }

    var body: some View {
        ForEach(names) { name in
            NameTile(of: name, for: list)
        }
    }
}

struct NameTile: View {
    @Environment(\.managedObjectContext)
    private var viewContext
    @FetchRequest
    private var items: FetchedResults<ListItemEntity>
    @ObservedObject
    private var list: ListEntity
    @ObservedObject
    private var name: NameEntity

    private var item: ListItemEntity? {
        items.first { $0.name == name }
    }

    init(of name: NameEntity, for list: ListEntity) {
        _list = ObservedObject(wrappedValue: list)
        _name = ObservedObject(wrappedValue: name)
        _items = FetchRequest(
            fetchRequest: ListItemEntity.fetchRequestFor(list),
            animation: .default
        )
    }

    var body: some View {
        Button {
            if let item {
                viewContext.delete(item)
            } else {
                let newItem = ListItemEntity(context: viewContext)
                newItem.flag = false
                newItem.name = name
                newItem.list = list
            }
            viewContext.forceSave()
        } label: {
            Label(name.wrappedName, systemImage: item != nil ? "checkmark.circle" : "circle")
        }
    }
}

I believe it must be something simple and obvious as this is very simple use case I think, but my lack of experience get my to point I can’t do anything more than ask for your help.