If you have ever worked on an iOS 5 application and came across a strange issue adding objects to a NSOrderedSet that is part of a NSManagedObject, you are not alone. This bug was reported on Open Radar on September 13, 2011, and was still reproducible in the latest versions of iOS5.

Setting up the Problem

To replicate the problem, create two NSManaged objects.

1. Create a NSManagedObject called “Student”. Add the attribute “name” of type string.
2. Create a NSManagedObject called “Lecture”. Add the attribute “name” of type string.
3. On your Student object, add the relationship “lectures”, and set the Destination to “Lecture”. Make sure the boxes are checked next to “To-Many relationship” and “Ordered”.
4. On your Lecture object, add the relationship “students”, and set the type to “Student”. Set the Inverse to “lectures”. Make sure the boxes are checked next to “To-Many relationship” and “Ordered”.

A student can have many lectures, and a lecture may have many students. We set the inverse relationship because we might need to know which students are in a particular lecture, as well as which lectures a particular student is enrolled in.

Replicating the Problem

To replicate this problem, create a Lecture and save it to CoreData. Next, create a student, and try to insert it into the “students” relationship in the lecture object you just created:

AppDelegate *delegate = [[UIApplication sharedApplication] delegate];

// Create the lecture
Lecture *lecture = [NSEntityDescription insertNewObjectForEntityForName:@"Lecture" inManagedObjectContext:delegate.managedObjectContext];
lecture.name = @"Obedience Training";

// Create the student
Student *student = [NSEntityDescription insertNewObjectForEntityForName:@"Student" inManagedObjectContext:delegate.managedObjectContext];
student.name = @"Rogan";

[lecture addStudentsObject:student];
[delegate saveContext];

What happens when you run this?

When you run this code, assuming you set the inverse relationship, you should get this exception:

‘NSInvalidArgumentException’, reason: ‘*** -[NSSet intersectsSet:]: set argument is not an NSSet’

How can I fix this?

One clean approach to fixing this issue is to add a category to your lectures model:

Note: one reason you should use a category – if you decide to make changes to your CoreData model and regenerate the class, it will not delete your category. If you placed this function in your model, re-generating the model will erase your code.

1. Select New>File.
2. In the left pane, select “Cocoa Touch”, and then select “Objective-C category”.
3. In the “Category on” box, select “Lecture”, and name the Category “methods” (it can be named whatever you like, but I am sticking with “methods” for this small example).

You will notice in your project explorer, a file will be created called “Lecture+methods.h”. This file will contain all of your extra methods for your object.

In your Lecture+methods.h file, override the addStudentsObject: method –

-(void)addStudentsObject:(Student *)value
{
     // Create a mutable set with the existing objects, add the new object, and set the relationship equal to this new mutable ordered set
     NSMutableOrderedSet *students = [[NSMutableOrderedSet alloc] initWithOrderedSet:self.students];
     [students addObject:value];
     self.students = students;
}

That’s it! Running the code should now work.