iPhone Core Data: NSFetchRequest with distinct properties and alphabet sections

Question

Hello everyone,

I am trying to create a NSFetchRequest that produces results for a UITableView.

It should find all the distinct occurrences of a NSString property and split them into alphabet sections ('A', 'B', 'C', etc.)

I have set up a method on my NSManagedObject class to return the first letter of the property:

- (NSString *)entrantFirstLetter
{
     [self willAccessValueForKey:@"entrantFirstLetter"];
     NSString *returnString = [self.entrant substringToIndex:1];
     [self didAccessValueForKey:@"entrantFirstLetter"];
     return returnString;
}

• I set 'sectionNameKeyPath' to @"entrantFirstLetter" and this works perfectly

• However, I now need to set returnsDistinctResults to YES

• But, returnsDistinctResults only works if propertiesToFetch is set, so

• I set propertiesToFetch to "entrant" (the property I'm interested in)

• But, in order for propertiesToFetch to work, the resultType must be NSDictionaryResultType, so

• I set resultType to NSDictionaryResultType

• But, this resultsType means that my 'sectionNameKeyPath' of @"entrantFirstLetter" no longer works.

So, you'd think the answer was to add in 'entrantFirstLetter' to the propertiesToFetch? But as it's not a property on the NSEntityDescription I can't!

All I want to do is sort a list of strings in Core Data into alphabet sections and not have duplicates. I can get each part working on its own, but getting it all working together relies on a seemingly endless loop of dependencies and I can't find a way to get it all working.

Any ideas would be much appreciated,

Russell.

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Project" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSDictionary *properties = [entity propertiesByName];
NSArray *propertiesToFetch = [NSArray arrayWithObject:[properties objectForKey:@"entrant"]];
[fetchRequest setResultType:NSDictionaryResultType];
[fetchRequest setPropertiesToFetch:propertiesToFetch];
[fetchRequest setReturnsDistinctResults:YES];
NSSortDescriptor *entrantDescriptor = [[NSSortDescriptor alloc] initWithKey:@"entrant" ascending:YES];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:entrantDescriptor]];

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:managedObjectContext sectionNameKeyPath:@"entrantFirstLetter" cacheName:nil];

Answer 1

You are trying to duplicate the NSFetchResultsController's default section behavior.

Instead, just set the sectionNameKeyPath: to entrant and the FRC will create the sections for you with no more effort on your part.

Answer 2

So, in the end this would not work at all with the NSFetchResultsController (this was under the 3.1.3 SDK). I just did my own NSFetchRequest in loadView that sorts all entrants into arrays according to their initial letter and at the same time stores the valid section header names.

Maybe Apple fixed the problem described in 4.x, but I haven't needed this functionality since or updated this project.