NSDecimalNumber sum - ios

Question

In my ios project I use two entities (CoreData): Person and Gifts with To-Many Relationship
I know how to calculate the sum of gifts to one person:
NSDecimalNumber *orderSum=[person.gifts valueForKeyPath:#"#sum.priceOfGift"];
But how do I calculate the sum of all persons?
-(NSFetchedResultsController *) fetchedResultsController {
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
//NSError *error = nil;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Person"
inManagedObjectContext:[self managedObjectContext]];
[fetchRequest setEntity:entity];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"nameOfPerson"
ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
_fetchedResultsController = [[NSFetchedResultsController alloc]initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:#"nameOfGroup" cacheName:nil];
_fetchedResultsController.delegate = self;
return _fetchedResultsController;
}

Answer 1

The "sum of all persons" would be to fetch all persons and count them
int sum = fetchedObjects.count;
But perhaps you mean the "sum of the prices of all gifts of all persons". If you think about it, it is the same as the sum of the prices of all gifts. Thus, you can just fetch the gifts and calculate
NSNumber *sum = [self.fetchedResultsController.fetchedObjects
valueForKeyPath:#"#sum.priceOfGift"];

Answer 2

You can try to fetch all the Persons, then use the sum
NSError *error = nil;
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = [NSEntityDescription entityForName:#"Person" inManagedObjectContext:managedObjectContext];
request.predicate = nil;
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"index" ascending:YES]];//some parameter
NSArray *persons = [managedObjectContext executeFetchRequest:request error:&error];
NSDecimalNumber *orderSum;
for (Person *person in persons)
orderSum = [NSDecimalNumber decimalNumberWithDecimal:orderSum.decimalValue
+[person valueForKeyPath:#"gifts.#sum.priceOfGift"].decimalValue]
Though, now work with NSDecimalNumber look a bit uncomfortable.