Core Data: NSFetchRequest sorting by count of to-many relationship

Say I have an parent entity, each of which have a number of children. I want to get all the parents sorted by their number of children. Something similar to the following pseudo code:

NSEntityDescription * entity = [NSEntityDescription entityForName:@"Parent" inManagedObjectContext:managedObjectContext];

[[NSSortDescriptor alloc] initWithKey:@"children.count" ascending:NO];
//Execute request

Is there a way construct a f开发者_StackOverflow社区etch like this using core data? If there is no way to do this will sorting using sortedArrayUsingSelector: loose the benefits of _PFBatchFaultingArray batch size?

Thanks, Ben

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Your query would work, but (assuming children is faulted) would use key-value coding methods on the children property, which in turn would fire the fault (see the NSManagedObject docs for a list of methods that fire faults, and a discussion of this behavior), so you'd lose the performance benefits of batching and faulting.

You might consider maintaining a derived attribute on your parent entity (call it childrenCount) that reflects the number of children related to the parent, if this is feasible for your situation. It's not the cleanest solution, but if you keep it as an NSNumber in the parent entity you'd have access to it even if children is faulted, and you can sort on it directly.