Question

My app will allow users to create a personalised list of events from a large list of events. I have a table view which simply displays these events, tapping on one of them takes the user to the event details view, which has a button “add to my events”.

In this detailed view I own the original event object, retrieved via an NSFetchedResultsController and passed to the detailed view (via a table cell, the same as the core data recipes sample). I have no trouble retrieving/displaying information from this “event”. I am then trying to add it to the list of MyEvents represented by a one to many (inverse) relationship:

alt text http://www.imgplace.com/img580/8008/44relationship.png

This code:

NSManagedObjectContext *context = [event managedObjectContext];
MyEvents *myEvents = (MyEvents *)[NSEntityDescription insertNewObjectForEntityForName:@"MyEvents" inManagedObjectContext:context];
[myEvents addEventObject:event];//ERROR

And this code (suggested below):

//would this add to or overwrite the "list" i am attempting to maintain
NSManagedObjectContext *context = [event managedObjectContext];
MyEvents *myEvents = (MyEvents *)[NSEntityDescription insertNewObjectForEntityForName:@"MyEvents" inManagedObjectContext:context];
NSMutableSet *myEvent = [myEvents mutableSetValueForKey:@"event"];
[myEvent addObject:event]; //ERROR

Bot produce (at the line indicated by //ERROR):

*** -[NSComparisonPredicate evaluateWithObject:]: message sent to deallocated instance

Seems I may have missed something fundamental. I cant glean any more information through the use of debugging tools, with my knowledge of them.

1) Is this a valid way to compile and store an editable list like this?

2) Is there a better way?

3) What could possibly be the deallocated instance in error?

—

I have now modified the Event entity to have a to-many relationship called “myEvents” which referrers to itself. I can add Events to this fine, and logging the object shows the correct memory addresses appearing for the relationship after a [event addMyEventObject:event];. The same failure happens right after this however. I am still at a loss to understand what is going wrong. This is the backtrace

#0  0x01f753a7 in ___forwarding___ ()
#1  0x01f516c2 in __forwarding_prep_0___ ()
#2  0x01c5aa8f in -[NSFetchedResultsController(PrivateMethods) _preprocessUpdatedObjects:insertsInfo:deletesInfo:updatesInfo:sectionsWithDeletes:newSectionNames:treatAsRefreshes:] ()
#3  0x01c5d63b in -[NSFetchedResultsController(PrivateMethods) _managedObjectContextDidChange:] ()
#4  0x0002e63a in _nsnote_callback ()
#5  0x01f40005 in _CFXNotificationPostNotification ()
#6  0x0002bef0 in -[NSNotificationCenter postNotificationName:object:userInfo:] ()
#7  0x01bbe17d in -[NSManagedObjectContext(_NSInternalNotificationHandling) _postObjectsDidChangeNotificationWithUserInfo:] ()
#8  0x01c1d763 in -[NSManagedObjectContext(_NSInternalChangeProcessing) _createAndPostChangeNotification:withDeletions:withUpdates:withRefreshes:] ()
#9  0x01ba25ea in -[NSManagedObjectContext(_NSInternalChangeProcessing) _processRecentChanges:] ()
#10 0x01bdfb3a in -[NSManagedObjectContext processPendingChanges] ()
#11 0x01bd0957 in _performRunLoopAction ()
#12 0x01f4d252 in __CFRunLoopDoObservers ()
#13 0x01f4c65f in CFRunLoopRunSpecific ()
#14 0x01f4bc48 in CFRunLoopRunInMode ()
#15 0x0273878d in GSEventRunModal ()
#16 0x02738852 in GSEventRun ()
#17 0x002ba003 in UIApplicationMain ()

solution

I managed to get to the bottom of this.

I was fetching the event in question using a NSFetchedResultsController with a NSPredicate which I was releasing after I had the results. Retrieving values from the entities returned was no problem, but when I tried to update any of them it gave the error above. It should not have been released.

oustanding part of my question

What is a good way to create this sub list from a list of existing items in terms of a core data model. I don’t believe its any of the ways I tried here. I need to show/edit it in another table view. Perhaps there is a better way than a boolean property on each event entity? The relationship idea above doesn’t seem to work here (even though I can now create it).

Cheers.

Answer 1

Assuming that you have named the MyEvents <--->> Event relationship as events

MyEvents *myEvents = (MyEvents *)[NSEntityDescription insertNewObjectForEntityForName:@"MyEvents" inManagedObjectContext:context];

NSLog(@"MYEVENTS: %@", myEvents);
NSLog(@"EVENT: %@", event);

// NSMutableSet to hold the events
NSMutableSet *events = [event mutableSetValueForKey:@"events"];

// Add the event to the set. Core Data takes care of the rest
[events addObject:event];

As per my comment, consider why you are creating a new Entity to manage a collection. NSFetchedResultsController and the NSFetchedResultsControllerDelegate protocol were designed to work closely with tableViews. Refer to Apple documentation on how to use these classes.