CoreData Memory management issue

Question

I'm using Coredata and have a Data Model with the following toMany relationships:

Rooms.isUsedBy -> Class many classes use a Room
Class.hasStudents -> Students many Students are in a Class

Given the Room I want to get all of the students that use the room. Here is the simplified code:

-(void) studentsinRoom:(Room*)aRoom {
    NSSet* roomStudents = [[NSSet alloc]init];
    for (SchoolClass* aClass in aRoom.isUsedBy) {
        roomStudents = [roomStudents setByAddingObjectsFromSet:aClass.hasStudents];
    }
    [roomStudents release];
}

roomStudents is computed correctly.
However when exiting the method I get a “EXC_BAD_ACCESS” error.

I'm debuggin this in the iPhone Simulator.

I'm pretty sure that I'm doing/not doing something correctly with the memory management. I do have a break set on "malloc_error_break" and it is not being hit.

Answer 1

Change it to something like:

- (void) studentsinRoom:(Room*)aRoom
{
    NSSet* roomStudents = nil;
    for (SchoolClass* aClass in aRoom.isUsedBy)
    {
        roomStudents = [roomStudents setByAddingObjectsFromSet:aClass.hasStudents];
    } 
}

The setByAddingObjectsFromSet: method returns an auto-released NSSet object, so you don't want to release it yourself. You're getting the EXC_BAD_ACCESS when the autorelease pool is drained and the object you already released is released a second time.

Just as a side note, the name hasStudents seems confusing, it sounds like it would be a boolean - I think just students would make the code easier to read for that to-many relationship.

Answer 2

You should be able to do this with Key-Value coding. For example:

roomStudents = [aRoom valueForKeyPath:@"@distinctUnionOfSets.isUsedBy.hasStudents"];

See the documentation for more.