An easy, reliable way to get max value of a core data object's attribute?

Is there an eas开发者_JS百科y (or just reliable) way to find the max value of a core data attribute? Apple's example just does not work (plus it is ridiculously long and complicated for such a simple task). I have spent almost a day on this and haven't been able to find a satisfactory answer. Please help!

I get the same error as in this question: -[NSCFNumber count]: unrecognized selector. Like him, I haven't been able to find a solution to the problem.

This asker thinks he solved the same problem but, like someone else commented, the answer here is apparently wrong.

This question also has trouble with exactly the same code but at least the asker actually didn't get at an exception. It appears that he couldn't get it to work properly though and ended up using a different solution but didn't say what.

One person here got around it by retrieving results sorted and using the top value. Not ideal! However, if I cannot find a solution soon, I think I will have to do the same, or restructure my model or business rules or create and maintain a MaxValue class in my model to get around this...

I found this in the core data programing guide under fetching specific values. It refers to a minimum, but it answers your question. Not as easy as one would hope, but it isn't that hard to follow.

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Event" inManagedObjectContext:context];
[request setEntity:entity];
// Specify that the request should return dictionaries.
[request setResultType:NSDictionaryResultType];
// Create an expression for the key path.
NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"creationDate"];
// Create an expression to represent the minimum value at the key path 'creationDate'
NSExpression *minExpression = [NSExpression expressionForFunction:@"min:" arguments:[NSArray  arrayWithObject:keyPathExpression]];
// Create an expression description using the minExpression and returning a date.
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
// The name is the key that will be used in the dictionary for the return value.     

[expressionDescription setName:@"minDate"];
[expressionDescription setExpression:minExpression];
[expressionDescription setExpressionResultType:NSDateAttributeType];
// Set the request's properties to fetch just the property represented by the expressions.
[request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];
// Execute the fetch.
NSError *error = nil;
NSArray *objects = [managedObjectContext executeFetchRequest:request error:&error];
if (objects == nil) {
    // Handle the error
}
else {
    if ([objects count] > 0) {
        NSLog(@"Minimum date: %@", [[objects objectAtIndex:0] valueForKey:@"minDate"]);
    }
}



[expressionDescription release];

[request release];

have a NSManagedObject Subclass called TestEntity, which has one double property called tesetID.

-(double)getNextIndex
{
    NSFetchRequest * request = [[NSFetchRequest alloc] init];
    [request setEntity:[NSEntityDescription entityForName:@"TestEntity" inManagedObjectContext:self.managedObjectContext]];
    NSError * err = nil;
    NSArray * array = [self.managedObjectContext executeFetchRequest:request error:&err];
    NSNumber * value = [array valueForKeyPath:@"@max.testID"];

    return [value doubleValue]+1;
}
-(void)test
{
 for (int i = 0; i<25 ; i++) {
        TestEntity * entity = [[TestEntity alloc] initWithEntity:[NSEntityDescription entityForName:@"TestEntity" inManagedObjectContext:self.managedObjectContext] insertIntoManagedObjectContext:self.managedObjectContext];
        entity.testID = [NSNumber numberWithDouble:[self getNextIndex]];
        NSLog(@"our testID is set as: %@",entity.testID);
    }

}