CODEWITHSUNDEEP
iphoneioscore-datansmanagedobjectcontext
vm2000
vm2000

Reputation: 342

CoreData relationships are nil after mergeChangesFromContextDidSaveNotification

Having some truly strange behavior with Core Data on iOS.

I have a NSManagedObjectContext for my main thread used to read data from a SQLLite persistent store and display it to the user. I also have background processes managed by an NSOperationQueue. These background processes create an NSMangedObjectContext, fetch data from remote servers, and persist that data to the local Core Data store.

I have registered for NSManagedObjectContextDidSaveNotification and when I receive those notifications I call mergeChangesFromContextDidSaveNotification on the main thread's NSManagedObjectContext (passing the notification object as an argument).

This is all very standard and the way that all the Core Data document suggest that you deal with multi-threading.

Up until recently, I have always been inserting new objects into the data store and not modifying objects in the data store. This worked fine. After a background thread writes new data, the merge occurs, I send a notification to the UIController and redraw my display. The display draws correctly.

Recently I have made a change, and the background thread is both inserting and modifying objects. But all the rest of the pattern remains the same. Now after the merge, the data in my main thread’s NSManagedObjectContext is corrupted. If I try to query for objects, I get nothing back. If I try to examine objects which I already have a reference to all their relationships are nil (not faults but nil). I have checked the SQLLite database and the data is all there.

The only solution seems to be to reset the NSManagedObjectContext which isn’t acceptable given the architecture of the applicaiton.

Ok, a final bit of strangeness. If my background thread is only updating attributes (primitves) then I don’t get this strange behavior. However, if I update the relationships themselves, then I get these empty fetch request results and nil’d out relationships.

What am I missing?

Upvotes: 3

Views: 1343

Answers (1)

vm2000
vm2000

Reputation: 342

Ok, I figured out my problem. My problem is the way that mergeChangesFromContextDidSaveNotification merges changes from one thread to another. As I have read several times, it does not do a playback of the changes you made but rather merges the final state together. I didn't realize the ramifications of this. In my background thread, I am deleting an object, but before I delete it I nil out some of its relationships because they are marked as cascade delete and I don't want them deleted. Apparently when I merge those deleted objects into the main context, Core Data still enforces the delete rules and cascade deletes the related objects in the main context. I changed my delete rules and now my relationships is not getting incorrectly nil'd out. It is unfortunate that I have a relationship which in some cases I want cascade deleted and in others I don't. I will have to work on that.

Upvotes: 3

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