Mogenerator with NSFetchedResultsController - 0 sections - objective-c

Question

I am populating a UITableView from Core Data using an NSFetchedResultsController. The data is coming from an NSManagedObject subclass generated by Mogenerator called MenuItem. The MenuItem entity has a SectionID parameter that is an NSNumber and this is used to determine what section the item should be in within the table view.
I performed a test fetch on the data and confirmed that Core Data was populated correctly. All was fine.
The NSFetchedResultsController is created as follows and the sectionKeyNamePath is set to #"sectionId":
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"MenuItem" inManagedObjectContext:self.persistenceController.moc];
[fetchRequest setEntity:entity];
fetchRequest.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:#"sectionId" ascending:YES], [NSSortDescriptor sortDescriptorWithKey:#"rowId" ascending:YES]];
NSFetchedResultsController *frc = nil;
frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:self.persistenceController.moc
sectionNameKeyPath:#"sectionId"
cacheName:nil];
The problem (and solution):
With this code, the sections are not identified. The NSFetchedResultsController always returns 0. This worked previously when I created the NSMO subclass manually, so I figured it was something related to Mogenerator.
If I change the sectionNameKeyPath to be #"primitiveSectionId" then it works.
Hopefully this will help somebody in the future, but I don't understand what is going on here. Please could somebody explain why this fixes the problem?
Thanks

Answer 1

Why are you setting the FRC to nil? Try this (it works for me using an NSNumber Attribute):
With mogenerator you can get the entity using "[MenuItem entityName]" and access attributes using MenuItemAttributes.sectionId, for example.
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:[MenuItem entityName]];
fetchRequest.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:MenuItemAttributes.sectionId ascending:YES], [NSSortDescriptor sortDescriptorWithKey:MenuItemAttributes.rowID ascending:YES]];
NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:self.persistenceController.moc
sectionNameKeyPath:MenuItemAttributes.sectionId
cacheName:nil];
// Make sure you set your frc as the delegate
frc.delegate = self;
NSError *error = nil;
if (![fetcher performFetch:&error]) {
/*
* Did you execute the request?
*/
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
NSLog(#"%lu", [frc.sections count]);