View Full Version : Core Data Migration Exception - [NSSymbolicExpression length]

Jul 3, 2012, 04:08 AM
I'm getting an exception during a core data migration -

-[NSSymbolicExpression length]: unrecognized selector sent to instance

Here's the relevant part of the stack trace:

Exception at stack trace: (
0 CoreFoundation 0x00007fff850807b4 __exceptionPreprocess + 180
1 libobjc.A.dylib 0x00007fff824190f3 objc_exception_throw + 45
2 CoreFoundation 0x00007fff850da110 +[NSObject(NSObject) doesNotRecognizeSelector:] + 0
3 CoreFoundation 0x00007fff8505291f ___forwarding___ + 751
4 CoreFoundation 0x00007fff8504ea68 _CF_forwarding_prep_0 + 232
5 Foundation 0x00007fff86b3dd23 -[NSObject(NSKeyValueCoding) valueForKeyPath:] + 57
6 Foundation 0x00007fff86ba0902 -[NSFunctionExpression expressionValueWithObject:context:] + 530
7 Foundation 0x00007fff86ba088d -[NSFunctionExpression expressionValueWithObject:context:] + 413
8 CoreData 0x00007fff813b8fc8 -[NSEntityMigrationPolicy createRelationshipsForDestinationInstance:entityMapping:manager:error:] + 1160
9 CoreData 0x00007fff813bcce7 -[NSMigrationManager(InternalMethods) _doSecondPassForMapping:error:] + 551
10 CoreData 0x00007fff813bbb70 -[NSMigrationManager migrateStoreFromURL:type:options:withMappingModel:toDestinationURL:destinationType:destinationOption s:error:] + 2224
11 CoreData 0x00007fff813b6ab3 -[NSStoreMigrationPolicy(InternalMethods) migrateStoreAtURL:toURL:storeType:options:withManager:error:] + 147
12 CoreData 0x00007fff813b68ca -[NSStoreMigrationPolicy migrateStoreAtURL:withManager:metadata:options:error:] + 106
13 CoreData 0x00007fff813b77cc -[NSStoreMigrationPolicy(InternalMethods) _gatherDataAndPerformMigration:] + 1116
14 CoreData 0x00007fff812f863d -[NSPersistentStoreCoordinator addPersistentStoreWithType:configuration:URL:options:error:] + 1997
15 AppKit 0x00007fff841c3101 -[NSPersistentDocument configurePersistentStoreCoordinatorForURL:ofType:modelConfiguration:storeOptions:error:] + 255
16 Beadia 0x0000000100004a6d -[BeadiaDocument configurePersistentStoreCoordinatorForURL:ofType:modelConfiguration:storeOptions:error:] + 525

The new model simply has a few extra fields, both transient and non-transient, in existing entities and a couple of new entities. I have a mapping model in place, and now this exception has appeared (the migration has been successful a few times now).

Any ideas as to how I can debug this? I've been through the mapping model and can't see anything wrong, there are no literal values and I've not made any changes. Google finds nothing relevant about this exception.

Jul 3, 2012, 10:19 AM
An "unrecognized selector" exception is often due to an over-release or under-retain. When that happens, the object-pointer is recycled, the pointer then points to a different class, and the old object-pointer now tries to run a method (e.g. length) on an object whose class (e.g. NSSymbolicExpression) doesn't have that method. This wouldn't be expected if you're using ARC, but since you didn't say, I'll guess you're not.

Another cause of "unrecognized selector" is a mistake in the presumed class of an object when KVC is being used. Since valueForKeyPath: is in the stack trace, and that's a KVC method, that's where you should probably start. You may be making an assumption somewhere that the class should be X when it's really Y. I don't know what X is, but it's something with a length method. Y is clearly NSSymbolicExpression.

And google finds quite a lot on "unrecognized selector sent to instance", and they seem quite relevant to me. Top result:
which mentions memory management issues.

Jul 3, 2012, 12:09 PM
Thanks, I think it's memory management too (I'm using GC :( ). I've narrowed the issue down to one specific entity - I set up an NSEntityMigrationPolicy subclass which outputs debug information in the createRelationshipsForDestinationInstance:entityMapping:manager:error: method which helps to show what's going on.

Jul 7, 2012, 09:18 AM
I finally found this, I think. I have an entity with a relationship called size, which is a Core Data reserved word. When XCode generated a mapping model it didn't prefix the size with # in the same way it does for a non-relationship attribute. I just added the # and everything seems okay.

It's only taken DAYS to find this... :rolleyes: