My Feature Request for NSPropertyDescription (Bug Reporter #11130505)

NSPropertyDescriptionTransientBlock.m

/*

Enhancement Request for Core Data
    Apple Radar Issue #11130505
    http://openradar.appspot.com/radar?id=3173403

I'd like to see an expansion of the Core Data framework that allows us to add a
transient property's underlying code via a block assigned on an 
NSPropertyDescription, in order to provide an easy means to use transient properties
without subclassing the managed object.

I create my Core Data model, entities, attributes, and relationships at runtime using 
a JSON model description.  However, in the event that I want to use an 
NSFetchedResultsController with sections based on (for example) a User object's 
lastName *initial* letter.  Right now, since I don't subclass the objects at design-
time, if I want to use sections in the FRC I have to include the actual letter value
in a separate "sectionBy" property, then make sure to change it's value whenever 
someone changes the User's lastName.

However, whenever I'm building the NSEntityDescription, it would be far cleaner if I 
merely needed to create an NSAttributeDescription (call it "sectionBy") and then set 
it up like follows: 
*/

+ (NSAttributeDescription *)transientSectionByLastNameInitial {
    NSAttributeDescription *transient = [[NSAttributeDescription alloc] init];
    [transient setName:@"sectionBy"];
    [transient setTransient:YES];
    [transient setAttributeType:NSUndefinedAttributeType];
    [transient setAttributeValueClassName:@"NSString"];

    // The Good Stuff
    [transient setTransientValueWithBlock:(id (^)(void)){
        NSString *input = [self valueForKey:@"lastName"];
        if (!input || !input.length)
            return nil;
        return [input substringToIndex:1];
    }];
    return transient;
}