Question

So i’m displaying some Entities in a UITableView. With clicking on a Cell i want to show other Entities that are already queried in a “to-many” Relationship.

For example i’m displaying all Classes of a School. Now i want to display all Students of a Class. This Students are already available as an NSSet under Class.students

Now i want to display the Students in different Sections following by their first Letter.

If i wanted to get them directly from CoreData, i would do something like

// init fetch request
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Student" inManagedObjectContext:self.managedObjectContext];

// Search only specific students
[fetchRequest setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"class == %@", theClassThoseStudentsBelongTo];
[fetchRequest setPredicate:predicate];

// Generate it
NSFetchedResultsController *theFetchedResultsController = 
    [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest 
                                        managedObjectContext:self.managedObjectContext 
                                          sectionNameKeyPath:@"firstLetter" 
                                                   cacheName:@"StudentTable"];

with this method i would get them nicely arranged into sections.

But i already have all students for a specific Class. is there a way to init a NSFetchedResultsController with a initialized NSSet or to do something equal?

Sure, i could arrange my NSSet manually but isn’t there such a nice way like it is for a new query?

thanks in advance.
Please leave a comment if something is unclear.

Answer 1

I guess you only have 2 options: using NSFetchedResultsController or sorting the objects on your own.

NSFetchedResultsController & NSPredicate:
Pros: easy object deletition; notifications of model changes, e.g. during syncing
Cons: unnecessary refetch

NSSet & NSSortDescriptor
Pros: no refetch
Cons: complicated deletition; no notifications of model changes, e.g during syncing: you could be displaying a student that has already been deleted