Core Data - iOS Programming: The Big Nerd Ranch Guide - page 496

The format string for a predicate can be very long and complex. Apple's Predicate Pro-
gramming Guide is a complete discussion of what is possible.
Predicates can also be used to filter the contents of an array. So, even if you had already
fetched the allItems array, you could still use a predicate:
NSArray *expensiveStuff = [allItems filteredArrayUsingPredicate:p];
Adding and deleting items
This handles saving and loading, but what about adding and deleting? When the user
wants to create a new BNRItem , you will not allocate and initialize this new BNRItem .
Instead, you will ask the NSManagedObjectContext to insert a new object from the
BNRItem entity. It will then return an instance of BNRItem . In BNRItemStore.m ,
edit the createItem method.
- (BNRItem *)createItem
{
BNRItem *p = [[BNRItem alloc] init];
double order;
if ([allItems count] == 0) {
order = 1.0;
} else {
order = [[allItems lastObject] orderingValue] + 1.0;
}
NSLog(@"Adding after %d items, order = %.2f", [allItems count], order);
BNRItem *p = [NSEntityDescription insertNewObjectForEntityForName:@"BNRItem"
inManagedObjectContext:context];
[p setOrderingValue:order];
[allItems addObject:p];
return p;
}
When a user deletes a BNRItem , you must inform the context so that it is removed from
the database. In BNRItemStore.m , add the following code to removeItem: .
- (void)removeItem:(BNRItem *)p
{
NSString *key = [p imageKey];
[[BNRImageStore sharedStore] deleteImageForKey:key];
[context deleteObject:p];
[allItems removeObjectIdenticalTo:p];
}