如何通过valueForKey从fetch中对数组进行排序

我有一个fetchResultController，它通过运行多个方法来编译数组 . 最后的单元格显示名称和编号（排名） . 我想从tableview中取出单元格并按数字（升序）对它们进行排序 . 我在frc尝试过nssortdescriptor但是在我尝试调用'排名'时会遇到问题，因为它不是我正在获取的实体的关键路径 . 有没有一个地方我可以截取这个数组来自frc，按valueForKey排序：@“排名”？我该如何实现这种方式？

它循环创建valueForKey的方法如下：

- (NSExpressionDescription*) rankingExpressionDescriptionForTags:(NSSet*)itemToTag
{
    NSPredicate* p2 = [NSPredicate predicateWithFormat:@"SUBQUERY(itemToTag,$t,$t.tagName IN %@).@count > 0",[itemToTag valueForKey:@"tagName"]];
    NSExpression* rankExpresion = [(NSComparisonPredicate*)p2 leftExpression];
    NSExpressionDescription* rankExpDesc = [[NSExpressionDescription alloc] init];
    rankExpDesc.name = @"ranking";
    rankExpDesc.expression = rankExpresion;
    rankExpDesc.expressionResultType = NSInteger64AttributeType;

    return rankExpDesc;
}

- (NSExpressionDescription*) objectIDExpressionDescription
{
    NSExpressionDescription* expDesc = [[NSExpressionDescription alloc] init];
    expDesc.name = @"objectID";
    expDesc.expressionResultType = NSObjectIDAttributeType;
    expDesc.expression = [NSExpression expressionForEvaluatedObject];
    return expDesc;
}

- (NSFetchRequest*) rankingRequestForItem:(Item*)item 
{
    NSFetchRequest* r = [NSFetchRequest fetchRequestWithEntityName:@"Item"];
    NSPredicate* p = [NSPredicate predicateWithFormat:@"SELF != %@",item.objectID];
    r.resultType = NSDictionaryResultType;


    r.propertiesToFetch = @[[self objectIDExpressionDescription],@"itemName",
                            [self rankingExpressionDescriptionForTags:[item mutableSetValueForKey:@"itemToTag"]]];


    r.predicate = p;
    r.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"itemName" ascending:YES]];


    return r;
}

和cell配置的frc在这里：

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSLog(@"selected item: %@, itemToTag: %@",selectedItem.itemName,[selectedItem.itemToTag valueForKey:@"tagName"]);
    NSFetchRequest *fetchRequest = [self rankingRequestForItem:selectedItem];
    // 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:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error]) {

        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _fetchedResultsController;
}


- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    NSDictionary *item = [self.fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = [NSString stringWithFormat:@"%@ - %@",[item valueForKey:@"itemName"],[item valueForKey:@"ranking"]];
}

谢谢您的帮助！