NSFetchedResultsController with sections created by first letter of a string

Learning Core Data on the iPhone. There seem to be few examples on Core Data populating a table view with sections. The CoreDataBooks example uses sections, but they're generated from full strings within the model. I want to organize the Core Data table into sections by the first letter of a last name, a la the Address Book.

I could go in and create another attribute, i.e. a single letter, for each person in order to act as the section division, but this seems kludgy.

Here's what I'm starting with ... the trick seems to be fooling the sectionNameKeyPath:

- (NSFetchedResultsController *)fetchedResultsController {
//.........SOME STUFF DELETED
    // Edit the sort key as appropriate.
    NSSortDescriptor *orderDescriptor = [[NSSortDescriptor alloc] initWithKey:@"personName" ascending:YES];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:orderDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];
    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = 
            [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest 
            managedObjectContext:managedObjectContext 
            sectionNameKeyPath:@"personName" cacheName:@"Root"];
//....
}

Dave DeLong's approach is good, at least in my case, as long as you omit a couple of things. Here's how it's working for me:

Add a new optional string attribute to the entity called "lastNameInitial" (or something to that effect).

Make this property transient. This means that Core Data won't bother saving it into your data file. This property will only exist in memory, when you need it.

Generate the class files for this entity.

Don't worry about a setter for this property. Create this getter (this is half the magic, IMHO)

// THIS ATTRIBUTE GETTER GOES IN YOUR OBJECT MODEL
- (NSString *) committeeNameInitial {
    [self willAccessValueForKey:@"committeeNameInitial"];
    NSString * initial = [[self committeeName] substringToIndex:1];
    [self didAccessValueForKey:@"committeeNameInitial"];
    return initial;
}


// THIS GOES IN YOUR fetchedResultsController: METHOD
// Edit the sort key as appropriate.
NSSortDescriptor *nameInitialSortOrder = [[NSSortDescriptor alloc] 
        initWithKey:@"committeeName" ascending:YES];

[fetchRequest setSortDescriptors:[NSArray arrayWithObject:nameInitialSortOrder]];

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

PREVIOUSLY: Following Dave's initial steps to the letter generated issues where it dies upon setPropertiesToFetch with an invalid argument exception. I've logged the code and the debugging information below:

NSDictionary * entityProperties = [entity propertiesByName];
NSPropertyDescription * nameInitialProperty = [entityProperties objectForKey:@"committeeNameInitial"];
NSArray * tempPropertyArray = [NSArray arrayWithObject:nameInitialProperty];

//  NSARRAY * tempPropertyArray RETURNS:
//    <CFArray 0xf54090 [0x30307a00]>{type = immutable, count = 1, values = (
//    0 : (<NSAttributeDescription: 0xf2df80>), 
//    name committeeNameInitial, isOptional 1, isTransient 1,
//    entity CommitteeObj, renamingIdentifier committeeNameInitial, 
//    validation predicates (), warnings (), versionHashModifier (null), 
//    attributeType 700 , attributeValueClassName NSString, defaultValue (null)
//    )}

//  NSInvalidArgumentException AT THIS LINE vvvv
[fetchRequest setPropertiesToFetch:tempPropertyArray];

//  *** Terminating app due to uncaught exception 'NSInvalidArgumentException',
//    reason: 'Invalid property (<NSAttributeDescription: 0xf2dfb0>), 
//    name committeeNameInitial, isOptional 1, isTransient 1, entity CommitteeObj, 
//    renamingIdentifier committeeNameInitial, 
//    validation predicates (), warnings (), 
//    versionHashModifier (null), 
//    attributeType 700 , attributeValueClassName NSString, 
//    defaultValue (null) passed to setPropertiesToFetch: (property is transient)'

[fetchRequest setReturnsDistinctResults:YES];

NSSortDescriptor * nameInitialSortOrder = [[[NSSortDescriptor alloc]
    initWithKey:@"committeeNameInitial" ascending:YES] autorelease];

[fetchRequest setSortDescriptors:[NSArray arrayWithObject:nameInitialSortOrder]];

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

I think I've got yet another option, this one uses a category on NSString...

@implementation NSString (FetchedGroupByString)
- (NSString *)stringGroupByFirstInitial {
    if (!self.length || self.length == 1)
        return self;
    return [self substringToIndex:1];
}
@end

Now a little bit later on, while constructing your FRC:

- (NSFetchedResultsController *)newFRC {
    NSFetchedResultsController *frc = [[NSFetchedResultsController alloc] initWithFetchRequest:awesomeRequest
            managedObjectContext:coolManagedObjectContext
            sectionNameKeyPath:@"lastName.stringGroupByFirstInitial"
            cacheName:@"CoolCat"];
    return frc;
}

This is now my favorite approach. Much cleaner/easier to implement. Moreover, you don't have to make any changes to your object model class to support it. This means that it'll work on any object model, provided the section name points to a property based on NSString

Here's how you might get it to work:

Add a new optional string attribute to the entity called "lastNameInitial" (or something to that effect).
Make this property transient. This means that Core Data won't bother saving it into your data file. This property will only exist in memory, when you need it.
Generate the class files for this entity.

Don't worry about a setter for this property. Create this getter (this is half the magic, IMHO)

- (NSString *) lastNameInitial {

     [self willAccessValueForKey:@"lastNameInitial"];

     NSString * initial = [[self lastName] substringToIndex:1];

     [self didAccessValueForKey:@"lastNameInitial"];

     return initial;

}

In your fetch request, request ONLY this PropertyDescription, like so (this is another quarter of the magic):

NSDictionary * entityProperties = [myEntityDescription propertiesByName];

NSPropertyDescription * lastNameInitialProperty = [entityProperties objectForKey:@"lastNameInitial"];

[fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:lastNameInitialProperty]];

Make sure your fetch request ONLY returns distinct results (this is the last quarter of the magic):
```
[fetchRequest setReturnsDistinctResults:YES];
```

Order your results by this letter:

NSSortDescriptor * lastNameInitialSortOrder = [[[NSSortDescriptor alloc] initWithKey:@"lastNameInitial" ascending:YES] autorelease];

[fetchRequest setSortDescriptors:[NSArray arrayWithObject:lastNameInitialSortOrder]];

execute the request, and see what it gives you.

If I understand how this works, then I'm guessing it will return an array of NSManagedObjects, each of which only has the lastNameInitial property loaded into memory, and who are a set of distinct last name initials.

Good luck, and report back on how this works. I just made this up off the top of my head and want to know if this works. =)

I like Greg Combs answer above. I've made a slight modification so that strings like "Smith" and "smith" can appear in the same section by converting the strings to upper case:

- (NSString *)stringGroupByFirstInitial {
    NSString *temp = [self uppercaseString];
    if (!temp.length || temp.length == 1)
        return self;
    return [temp substringToIndex:1];
}

NSFetchedResultsController with sections created by first letter of a string

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