Calculating the max value for a core data attribute - NSCFNumber error

Question

Hi,

I need to find out the maximum value of an attribute of a core data entity.

I'm still firmly in the Cocoa learning curve, and this is a simple test app that I'm using to learn.

The app imports fortunes from a text file and displays a table on the screen. The imports are done in a separate, background thread.

I found this code online, which I have attempted to get working:

_- (double)getMaxID
{
NSLog(@"in getMaxID"); // debug

// Use a new moc with the original persistentStoreCoordinator to ensure thread safety
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init];
[moc setPersistentStoreCoordinator:[[self delegate] persistentStoreCoordinator]];

// Create fetch
NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
[fetch setEntity:[NSEntityDescription entityForName:@"Fortune" inManagedObjectContext:moc]];
[fetch setResultType:NSDictionaryResultType];

// Expression for Max ID
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"id"];
NSExpression *minExpression = [NSExpression expressionForFunction:@"max:" arguments: [NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
[expressionDescription setName:@"maxID"];
[expressionDescription setExpression:minExpression];
[expressionDescription setExpressionResultType:NSDoubleAttributeType];
[fetch setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];

// Execute the fetch.
double theID = 0;
NSError *error = nil;
NSArray *objects = nil;
objects = [moc executeFetchRequest:fetch error:&error]; // crashes here

if (objects && ([objects count] > 0))
{
NSLog(@"query successful"); // debug
theID = [((NSNumber *)[[objects objectAtIndex:0] valueForKey:@"maxID"]) doubleValue];
}
else
{
NSLog(@"Setting default value for theID"); // debug
theID = 0;
}

return(theID);
}

My entity is called "Fortune" and the attribute is called "id" (a Double).

When the code runs it crashes when the fetch request is executed. The console shows this:

2009-12-18 00:53:42.777 FortunesHelperMVC[4027:1703] -[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10
2009-12-18 00:53:42.778 FortunesHelperMVC[4027:1703] An uncaught exception was raised
2009-12-18 00:53:42.778 FortunesHelperMVC[4027:1703] -[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10
2009-12-18 00:53:42.779 FortunesHelperMVC[4027:1703] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFNumber count]: unrecognized selector sent to instance 0x1004d7b10'
* Call stack at first throw:
( 0 CoreFoundation 0x00007fff83ed9444 exceptionPreprocess + 180
1 libobjc.A.dylib 0x00007fff85fbb0f3 objc_exception_throw + 45
2 CoreFoundation 0x00007fff83f321c0 +[NSObject(NSObject) doesNotRecognizeSelector:] + 0
3 CoreFoundation 0x00007fff83eac08f ___forwarding_ + 751
4 CoreFoundation 0x00007fff83ea81d8 _CF_forwarding_prep_0 + 232
5 Foundation 0x00007fff88a5609e +[_NSPredicateUtilities max:] + 46
6 Foundation 0x00007fff8893ce72 -[NSFunctionExpression expressionValueWithObject:context:] + 530
7 CoreData 0x00007fff8613b5b1 -[NSMappedObjectStore executeFetchRequest:withContext:] + 2081
8 CoreData 0x00007fff8613ad10 -[NSMappedObjectStore executeRequest:withContext:] + 80
9 CoreData 0x00007fff86108900 -[NSPersistentStoreCoordinator(_NSInternalMethods) executeRequest:withContext:] + 688
10 CoreData 0x00007fff8610621b -[NSManagedObjectContext executeFetchRequest:error:] + 267
11 FortunesHelperMVC 0x0000000100001d9d -[ImportOperation getMaxID] + 572
12 FortunesHelperMVC 0x0000000100001f95 -[ImportOperation main] + 330
13 Foundation 0x00007fff888f406d -[__NSOperationInternal start] + 681
14 Foundation 0x00007fff888f3d23 ____startOperations_block_invoke_2 + 99
15 libSystem.B.dylib 0x00007fff86a98ce8 _dispatch_call_block_and_release + 15
16 libSystem.B.dylib 0x00007fff86a77279 _dispatch_worker_thread2 + 231
17 libSystem.B.dylib 0x00007fff86a76bb8 _pthread_wqthread + 353
18 libSystem.B.dylib 0x00007fff86a76a55 start_wqthread + 13
)
terminate called after throwing an instance of 'NSException'

Any ideas why this isn't working? I'm stumped after a lot of Googling.

Thanks

Darren.

Answer 1

The issue was that this line is wrong:

[fetch setResultType:NSDictionaryResultType];

The result type should have been NSManagedObjectResultType

Regards

Darren.