Question

I have an entity called LogBook which has an attribute (called columns) for a set of LogBookColumn entities (one-to-many relationship).

The standard way I see to retrieve the mutable set of columns seems to be:

NSEntityDescription *myLogbook;
myLogbook = [NSEntityDescription insertNewObjectForEntityForName:@"LogBook"
                                          inManagedObjectContext:self.managedObjectContext];
NSMutableSet *columns = [myLogbook mutableSetValueForKey:@"columns"];

Instead of method on the third line, I want to use dot notation. To do so, I have created class definitions, also called LogBook and LogBookColumn, and use @property to create the setters and getters.

LogBook *myLogBook;
myLogbook = [NSEntityDescription insertNewObjectForEntityForName:@"LogBook"
                                          inManagedObjectContext:self.managedObjectContext];
NSMutableSet *columns = (NSMutableSet *)myLogbook.columns;

So, is columns truly a mutable set by default? I have done two things to verify:

• Attempted to write to the list, eg: [columns addObject:aColumn];
• Asked with: BOOL isKindOfMutableSet = [myLogbook.columns isKindOfClass:[NSMutableSet class]];

Both work with expected results, which may make this question overkill, but I am very concerned about memory errors that will be difficult to track down. I also wonder if asking the question isKindOfClass will work as I have defined this as a mutable set – so won’t it work even if the underlying memory organization doesn’t support mutable sets?

All of the above sums up to: is this the right way to access and change the columns property/attribute?

Answer 1

According to this documentation, to-many relationships should be declared as NSSet, which makes sense. Even if the attribute returns an NSMutableSet, it is not guaranteed that updating the NSMutableSet will properly update relationships (which mutableSetValueForKey: does).

If you really want a mutable accessor, then just create a readonly property that wraps mutableSetValueForKey:

Update:

Otherwise, we are supposed to use – (void)addLogBookColumnsObject:(LogBookColumn *)value;, – (void)removeLogBookColumnsObject:(LogBookColumn *)value;, – (void)addLogBookColumns:(NSSet *)values;, – (void)removeLogBookColumns:(NSSet *)values; that were generated by Core Data for us.