Fetch one-to-many relationship fault still shows after adding setRelationshipKeyPathsForPrefetching - core-data

Question

Let's say my data model has two entities: Department and Person. Departments has a to-many relationship called 'departmentToPerson' with Person. Person has a to-one relationship called 'personToDepartment' with Department.
I want to populate an array of people belonging to a selected department. To selected the department I've created a UILabel that displays a departmentName that is selected from a popup tableview. When I run the app the log shows:
personToDepartment.departmentName == (entity:
Department; id: 0x8cc1920
;
data: {
departmentName = Marketing;
departmentToPerson = "";
I've read the purpose of faults and implemented setRelationshipKeyPathsForPrefetching, but still I get the fault. I'll put in the disclaimer that I'm a newbie and may be missing something obvious. When I delete the predicate my table view populates with all personName. Here's the code:
- (NSFetchedResultsController *)fetchedResultsController {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"Person" inManagedObjectContext:_managedObjectContext];
[fetchRequest setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"personToDepartment.departmentName = %#", selectedDepartment];
NSLog(#"predicate is: %#",predicate);
[fetchRequest setPredicate:predicate];
[fetchRequest setRelationshipKeyPathsForPrefetching:[NSArray arrayWithObjects: #"departmentToPerson", nil]];
fetchRequest.returnsObjectsAsFaults = NO;
NSSortDescriptor *sort = [[NSSortDescriptor alloc]
initWithKey:#"personName" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
[fetchRequest setFetchBatchSize:20];
NSFetchedResultsController *theFetchedResultsController =
[[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:_managedObjectContext sectionNameKeyPath:nil
cacheName:nil];
self.fetchedResultsController = theFetchedResultsController;
_fetchedResultsController.delegate = self;
return _fetchedResultsController;
}
Again, I select a new department from the UILabel, the log displays selectedDepartment, but then states relationship fault and does not populate the table.
Thanks in advance for the help!
Updated 9JUNE
I've also found this works:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"personToDepartment = %#", selectedDepartment];

Answer 1

I'm also kind of newbie to CoreData, but it seems to me that predicate is wrong. According to Log record you're about to comparing personToDepartment.departmentName to a department Object. Predicate should look like:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"personToDepartment.departmentName = %#", selectedDepartment.departmentName];
But there is a better way. selectedDepartment.departmentToPerson will return an NSSet with all persons belonging to this department (if relationship was set previously). But warning, if you'll try to NSLog(#"%#", selectedDepartment.departmentToPerson) probably you'll get "relationship fault", because CoreData will not do fetch until you address specific object in NSSet or enumerate through it. But for example NSLog(#"%d", selectedDepartment.departmentToPerson.count) will force CoreData to make fetch and you'll see number of persons in department.
UPDATE:
NSSet is empty probably because you are not setting relationship, when creating person's object. Inverse To-Many relationship from department to persons will be set automatically.
- (id)insertNewPerson:(NSDictionary *)personInfo inDepartment:(Department*)departmentObject
{
Person *personObject = [NSEntityDescription insertNewObjectForEntityForName:#"Person" inManagedObjectContext:context];
personObject.name = personInfo.name;
personObject.departmentName = departmentObject.name;
...
personObject.personToDepartment = departmentObject; //this is RELATIONSHIP, if set, then you can access all persons from specific department by doing: departmentObject.departmentToPerson
}