Crash when deleting object from Core Data and toggling view controllers

Question

I am making a NSFetchRequest for a NSManaged Object on my initial screen. I sometimes have a crash in a scenario when I :

• switch to another view controller within my tab bar controller
• make another fetch request with the same managed object type
• delete a common managed objects which also appears in my initial VC's fetchrequest. The VC contains a table view.
• save the managed context
• toggle to the first VC, and reload the data

I am not using NSFetchResutltsController to manage these returned objects. The crash happens when my tableview reloads. I do make another request, and expected the deleted objects not be returned, but it does. When my cells are trying to read a property of the deleted object, it reads uninitialized and crashes. This happens about 1 out of 5 times when toggling between the 2 VCs. I am using performAndWait in all of my CoreData functions.

Is there a way to decouple the the relationship of the Managed Objects between the two screens? If not, how can I get my fetch request in the first VC, not return the objects that were deleted in the second VC, keeping them in sync?

Answer 1

A NSManagedObject is not like other other objects. It does not contain any information itself. It has a pointer to its context and an objectID. When you access it's properties it forwards the request to the context to get the information that it needs. So when an entity is deleted from the context the managedObject stops working and causes a crash. This is why in general I think it is a bad practice to EVER keep a pointer to a managedObject and ALWAYS access them using a fetchedResultsController even if only for one object, and only do a fetch if the managedObjects results are discards right afterwards.

There are two possible solutions, which you hinted to in your question. Either you can copy the values out of the managedObject, or you can use a fetchedResultsController. If you copy the values then it will appears as normal even after the entity is deleted. If you use a fetchedResultsController then the fetchedObjects property will be never contain deleted object, and the object will be inaccessible after it is deleted.

I would recommend using a fetchedResultsController. You don't need to be afraid of it. It is not a large overhead and it reasonable to use even if you are fetching only one object.