Core data, sorting one-to-many child objects

Question

So if I have a store of parents children and the parent has a one to many relationship to children (parent.children) and they all have first names. Now, on the initial fetch for parents I can specify a sort descriptor to get them back in order of first name but how can I request the children in order? If I do a [parent.children allObjects] it just gives them back in a jumble and I'd have to sort after the fact, every time.

Thanks, Sam

Answer 1

Sam,

If I read your question correctly, you want to set up a fetch that returns a sorted list of the children of a specific parent. To do this, I would set up a fetch for "children" entities and then use a predicate to limit the results:

NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
[request setEntity:[NSEntityDescription entityForName:@"children" inManagedObjectContext:moc]];
[request setSortDescriptors:[NSArray initWithObject:[[NSSortDescriptor alloc] initWithKey:@"firstName" ascending:YES]];
[request setPredicate:[NSPredicate predicateWithFormat:@"(parent == %@)", parent]];

Obviously, your entity and attribute names may be different. In the last line, the parent variable should be a reference to the NSManagedObject instance of the parent whose children you want.

Answer 2

If you just want to use an NSArray, and not an NSFetchedResultsController, there's another way:

NSSortDescriptor *alphaSort = [NSSortDescriptor sortDescriptorWithKey:@"firstName" ascending:YES];
NSArray *children = [[parent.children allObjects] sortedArrayUsingDescriptors:[NSArray arrayWithObject:alphaSort]];