IOS Core Data Cardinality issue - ios

Question

I'm writing a simple application using TableViews and Core Data that shows a list of students in the first level and when clicked on a cell, it shows the names of the courses that student takes. I set student->courses as toMany, courses->student as toOne. This is how i pass the Student entity to course view controller for using its managedObjectContext:
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
Student *info = [_fetchedResultsController objectAtIndexPath:indexPath];
CoursesTableViewController *courseViewController = [[CoursesTableViewController alloc] initWithStudentInfo:info];
[self.navigationController pushViewController:courseViewController animated:YES];
}
This is how i create courses inside the courses view controller(I want only three courses as named below, numberofrow is supposed to assure that):
- (void)viewDidLoad
{
[super viewDidLoad];
id <NSFetchedResultsSectionInfo> sectionInfo = [self.fetchedResultsController sections][0];
int numberofrow = [sectionInfo numberOfObjects];
NSLog(#"%d",numberofrow);
NSLog(#"%d",[self.fetchedResultsController sections].count);
if(numberofrow <=2){
NSLog(#"creating courses");
[self insertNewObject:#"comp319"];
[self insertNewObject:#"comp314"];
[self insertNewObject:#"comp316"];
}
}
- (void)insertNewObject:(NSString *) str{
NSManagedObjectContext *context = [self.fetchedResultsController managedObjectContext];
NSEntityDescription *entity = [[self.fetchedResultsController fetchRequest] entity];
NSManagedObject *newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:[entity name] inManagedObjectContext:context];
[newManagedObject setValue:str forKey:#"course_name"];
NSError *error = nil;
if (![context save:&error]) {
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
}
How i create the fetchedResultsController inside courses View Controller:
- (NSFetchedResultsController *)fetchedResultsController
{
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
// Edit the entity name as appropriate.
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Courses" inManagedObjectContext:self.studentInfo.managedObjectContext];
[fetchRequest setEntity:entity];
// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];
// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"course_name" ascending:NO];
NSArray *sortDescriptors = #[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
// Edit the section name key path and cache name if appropriate.
// nil for section name key path means "no sections".
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.studentInfo.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
return _fetchedResultsController;
}
Finally the problem is that when I create the first student and click on it, NSLog in viewDidLoad of courses view controller above prints "creating courses", however when i create the second student it does not and also numberofrow prints 3 where it should have printed 0 since it is a new student instance.
Thanks for any help.

Answer 1

Replace
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Courses" inManagedObjectContext:self.studentInfo.managedObjectContext];
[fetchRequest setEntity:entity];
With
// my comment : entity here has to be your student name, why 'course'
// and also replace your search key from 'yourCourseNameKey' with 'yourStudentNameKey'
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Your Student Name" inManagedObjectContext:self.studentInfo.managedObjectContext];
[fetchRequest setEntity:entity];
You need to get number of Students but you query number of Courses

Answer 2

You do not set a predicate on the fetch provided to the fetched request controller so you aren't searching for courses for the selected student and creating then if they aren't there. Not that that is ideal anyway (and wouldn't work because you aren't setting a relationship between the student and all of the courses).
So, most likely you should have a set of courses and a set of students. The relationship should be many:many as a course really has multiple students and each student takes multiple courses (or could anyway).
Now, your students and courses would be created 'up-front' and then your view controllers simply deal with fetching, displaying and connecting (establishing the relationships) the objects.