Hi,
I need to find out the maximum value of an attribute of a core data
entity. I'm still 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];
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?
Thanks
Darren.
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