disordered/ordered context fetching

Question

Hello to everyone,

I cannot understanding why values assign to array objects become "disordered" after executing executeFetchRequest. Well, I'm not certain that fetching is the problem here. Here's the code.

objects=[NSArray arrayWithObjects:@"a",@"b",@"c",@"d",@"e",nil];

NSManagedObject *DB = nil;

for (int i=0;i<[objects count];i++){

DB = [NSEntityDescription      
    insertNewObjectForEntityForName:@"dbTable" 
    inManagedObjectContext:context];

[DB setValue:[objects objectAtIndex:i] forKey:@"item"];
[DB setValue:[NSString stringWithFormat:@"%3.2f", 0] forKey:@"value"];    
}

objects = [context executeFetchRequest:request error:&error];

Printings

new wrong ordering after fetch execution into array objects
index 0 :: Item: d
index 1 :: Item: c
index 2 :: Item: e
index 3 :: Item: b
index 4 :: Item: a

thank you.

Answer 1

When you execute a fetch request without specifying that you want your items sorted by a specific key, you get a result array which is not ordered. You can extend your fetch request with a sort descriptor using a code snippet such as the following one.

    // Edit the sort key as appropriate.
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"value" ascending:YES selector:@selector(caseInsensitiveCompare:)];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];
    [sortDescriptor release];
    [sortDescriptors release];

Answer 2

Apply an NSSortDescriptor to your fetch request, e.g.:

NSSortDescriptor *itemDescriptor = [[NSSortDescriptor alloc] initWithKey:@"item" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:itemDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[itemDescriptor release];

will sort on the item name. You could use value, instead.

Answer 3

Due to the underlying database architecture, Core Data stores all objects and to-many references in an unordered fashion. The order you get back from a fetch request is not guaranteed, unless you apply an NSSortDescriptor, as Alex describes. This thread in the Cocoa mailing list details the issues involved, as well as some potential solutions.

These solutions include having an index property that you sort on when performing a fetch request, or implementing a linked list of managed objects and traversing that in order. Brian Webster has created a subclass of NSManagedObject that lets you maintain ordered to-many relationships in your Core Data objects, for which the source code can be found here.