瞬态属性和 FetchRequest？

Question

我在 Book Entity 中有一个临时属性 titleFirstLetter：

- (NSString *)titleFirstLetter 
{
    NSString *tmpValue = nil;

    [self willAccessValueForKey:@"titleFirstLetter"];

    if ([[self title] length] > 0) {
        tmpValue = [[[self title] substringToIndex:1] uppercaseString];
        if ([[NSScanner scannerWithString:tmpValue] scanInt:NULL]) { //return # if its a number
            tmpValue = @"#";
        }
    } else { //sanity in case the attribute is not set.
        tmpValue = @"";
    }

    [self didAccessValueForKey:@"titleFirstLetter"];

    return tmpValue;
}

我正在尝试将此属性用作部分名称，但是当我执行时：

 // Create the sort descriptors array.
NSSortDescriptor *authorDescriptor = [[NSSortDescriptor alloc] initWithKey:@"titleFirstLetter" ascending:YES];
NSSortDescriptor *titleDescriptor = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:authorDescriptor, titleDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];

// Create and initialize the fetch results controller.
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"titleFirstLetter" cacheName:nil];
self.fetchedResultsController = aFetchedResultsController;
fetchedResultsController.delegate = self;

我收到此错误：

*** Terminating app due to uncaught exception 'NSInvalidArgumentException',  
reason: 'keypath titleFirstLetter not found in entity <NSSQLEntity Book id=1>'

请帮忙！

Answer 1

如果你只想要section index，而不想要section headers，你可以只传递title属性，它会实现你想要的。

sectionNameKeyPath:@"title"

否则，如果您确实想要只有第一个字母的部分标题，请参阅this previous question 进行完整讨论。