Why don't I have to release managedObjectContext in the 2nd TableViewController.

Question

Hi all. I have two tableview controllers showing CoreData objects. One is a detail view (with sentences) one is an overview (with stories). Pick a Story -> See the Sentences.

It looks like I was over-releasing my managedObjectContext; I originally released it in both TableViewControllers in dealloc and got a crash every third time I went between the two controllers (Story -> Sentence -> Story -> Sentence -> Story -> Crash). Some debugging showed I was crashing in my App Delegate after this code in ViewDidLoad of both TableViewControllers:

 if (managedObjectContext == nil) 
{ 
    managedObjectContext = [(StoryBotAppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
    NSLog(@"After managedObjectContext: %@",  managedObjectContext);
}

Some more research found this discussion which led me to believe it was a case of an over released ManagedObjectContext:

The second more prosaic issue is simply an overreleased NSManagedObject. Instruments ObjectAlloc tool should be able to help you.

So I removed [managedObjectContext release]; from my dealloc in TableViewController and now I have no leaks (according to Instruments) and no crash.

It looks like the problem is fixed, but here's the question:

• I might be missing the point altogether, and just hiding another issue. How can I find the over-release, or the real problem?

• If I have fixed the problem, I'd like to know why it's fixed and why I don't need to release the MOC in the second TableViewController

MakeSentenceTableViewController.m

@implementation MakeSentenceTableViewController
@synthesize story, managedObjectContext;
- (void)viewDidLoad {
[super viewDidLoad];

self.title = @"My Story";
NSLog(@"Passed Story Object: %@", story);
if (managedObjectContext == nil) 
{ 
    NSLog(@"managedObjectContext == nil");
    managedObjectContext = [(StoryBotAppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext];
    NSLog(@"After managedObjectContext: %@",  managedObjectContext);
}else{
    NSLog(@"managedObjectContext != nil");
}
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Sentence" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

//sorting stuff:
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"order" ascending: YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects: sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
//[request setFetchBatchSize:FETCH_BATCH_SIZE];
[sortDescriptors release];
[sortDescriptor release];

 fetchedResultsController = [[NSFetchedResultsController alloc] 
                            initWithFetchRequest:request managedObjectContext:managedObjectContext 
                            sectionNameKeyPath:nil cacheName:nil];
[request release];

NSError *error;
[fetchedResultsController performFetch:&error];

NSLog(@"FetchedResultsController: %@", fetchedResultsController);
NSLog(@"fetchedResultsController RetainCount at viewDidLoad: %d",     [fetchedResultsController retainCount]);
 }
//snip...table view bits
- (void)dealloc {
[fetchedResultsController release];
//Why don't I have to release this?
//[managedObjectContext release];
    [super dealloc];
}

Answer 1

Because you are not retaining it. Even if the "MOC" property in you view controller is (retain), you are not calling the setter but simply setting the reference directly. If you want to retain and release it, you have to call self.managedObjectContext = ... instead (notice the dot), that is equivalent to [self setManagedObjectContext:...] and only then you can safely release it in the dealloc. Actually, since the "MOC" is owned by and managed in the app delegate, I wouldn't even bother to retain it.