NSExpression is always returning zero - core-data

Question

I have an entity called Rounds that has basic data about golf rounds. I am trying to calculate the number of rounds as well as the average score. However, every time I try and calculate these values it returns 0 (zero). There are no errors, and no crashes.
I have the following function in Rounds.m:
+(NSNumber *)aggregateOperation:(NSString *)function onAttribute:(NSString *)attributeName withPredicate:(NSPredicate *)predicate inManagedObjectContext:(NSManagedObjectContext *)context
{
NSExpression *ex = [NSExpression expressionForFunction:function
arguments:[NSArray arrayWithObject:[NSExpression expressionForKeyPath:attributeName]]];
NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
[ed setName:#"result"];
[ed setExpression:ex];
[ed setExpressionResultType:NSInteger64AttributeType];
NSArray *properties = [NSArray arrayWithObject:ed];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setPropertiesToFetch:properties];
[request setResultType:NSDictionaryResultType];
if (predicate != nil)
[request setPredicate:predicate];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Rounds"
inManagedObjectContext:context];
[request setEntity:entity];
NSArray *results = [context executeFetchRequest:request error:nil];
NSDictionary *resultsDictionary = [results objectAtIndex:0];
NSNumber *resultValue = [resultsDictionary objectForKey:#"result"];
return resultValue;
}
I then call this method from my view controller to set the values of the labels for number of rounds and scoring average:
-(NSNumber*) scoringAverageCalc
{
NSNumber *scoreAverage = [Rounds aggregateOperation:#"average:" onAttribute:#"roundScore" withPredicate:nil inManagedObjectContext:_managedObjectContext];
return scoreAverage;
}
-(NSNumber*)countOfRounds
{
NSNumber *roundCount = [Rounds aggregateOperation:#"count:" onAttribute:#"roundDate" withPredicate:nil inManagedObjectContext:_managedObjectContext];
return roundCount;
}
Can someone please tell me why I am not getting the correct value?

Answer 1

I think NSExpression is overkill for simple sums. I would do it like this: fetch your rounds as normal managed objects (NSManagedObjectResultType) and then use KVC which should have just the aggregators you need.
NSNumber *sum = [rounds valueForKeyPath:#"#sum.score"];
NSNumber *avg = [rounds valueForKeyPath:#"#avg.score"];
Simple, isn't it? Check it out here.

Answer 2

Your code works for me. Are you sure there are no misspellings in your entity or attribute names?