Question

I have 2 views that contain a UITableView each. They are both displayed at the same time, side by side, on an iPad.

I am using Core Data for all data. Both tables need to be edited (rows added, deleted, etc), so I’d like to use a NSFetchedResultsController in each view to handle all this for me.

The contents of the second table depend on what is selected in the first table. So, when selecting an item in the first table, that object is passed to the view with the second table (so I do already have access to the data that should go into the second table), but I’d like to try to use all the built-in handling of the NSFRC if possible.

The model is along the lines of: University (uniID, uniName, students) and Student (stuID, stuName, university). So the relationship is: University <–>> Student.

I’m using the following code in the NSFRC, but it’s returning 0 results:

- (NSFetchedResultsController *)fetchedResultsController {

if (fetchedResultsController != nil) {
    return fetchedResultsController;
}

NSManagedObjectContext *context = appDelegate.managedObjectContext;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Student" inManagedObjectContext:context];
[fetchRequest setEntity:entity];

NSSortDescriptor *sort = [[NSSortDescriptor alloc]initWithKey:@"stuName" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

[fetchRequest setFetchBatchSize:20];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"university == %@",self.selectedUniversity];
[fetchRequest setPredicate:predicate];

NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];

self.fetchedResultsController = theFetchedResultsController;
fetchedResultsController.delegate = self;

[sort release];
[fetchRequest release];
[theFetchedResultsController release];

return fetchedResultsController;    

}

I would be most grateful if someone could at least point me in the right direction…

Answer 1

Ok, so I solved the issue. The NSFRC wasn’t being updated when the predicate term needed to change (i.e. a University had been selected), of course, because of:

if (fetchedResultsController != nil) {
    return fetchedResultsController;
}

Having the NSFRC recreated every time it’s called (by removing the above code) doesn’t work either, because the fetch needs to be executed after it’s been created, which can’t happen just before numberOfRowsInSection is called (as this method calls, and therefore recreates, the NSFRC).

So, I added a BOOL to the view called newFetchRequired which is set to YES every time a new University is selected. In the NSFRC, the above code should be changed to:

if (fetchedResultsController != nil && !newFetchRequired) {
        return fetchedResultsController;
    }
newFetchRequired = NO;

The fetch is then performed correctly (which calls and recreates the NSFRC):

[self.fetchedResultsController performFetch:&error];

I hope this helps anyone in a similar situation.

Thanks to Ashley for the alternative suggestion.