8

我有一个包含书籍和作者的核心数据项目。在数据模型中，Authors 与 Books 是一对多的关系，Books 与 Authors 是一对一的关系。我正在尝试提取所有没有作者的书籍。无论我如何尝试，都不会返回任何结果。在我的谓词中，我也尝试过 = NIL，== nil，== NIL。任何建议，将不胜感激。

// fetch all books without authors
- (NSMutableArray *)fetchOrphanedBooks {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Book" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
[fetchRequest setFetchBatchSize:20];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"author == nil"];
[fetchRequest setPredicate:predicate];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];

NSString *sectionKey = @"name";//nil;
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext
                                                                                                  sectionNameKeyPath:sectionKey cacheName:nil];
BOOL success = [aFetchedResultsController performFetch:nil];
NSMutableArray *orphans = nil;

// this is always 0
NSLog(@"Orphans found: %i", aFetchedResultsController.fetchedObjects.count);

if (aFetchedResultsController.fetchedObjects.count > 0)
{
   orphans = [[NSMutableArray alloc] init];
   for (Book *book in aFetchedResultsController.fetchedObjects)
   {
      if (book.author == nil)
      {
         [orphans addObject:book];
      }
   }

}

[aFetchedResultsController release];
[fetchRequest release];
[sortDescriptor release];
[sortDescriptors release];

return [orphans autorelease];

}
4

2 回答 2

26

尝试使用零计数：

NSPrdicate *predicate = [NSPredicate predicateWithFormat:@"author == nil || author.@count =0"];
于 2010-05-01T00:55:50.123 回答
0

尝试：

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"author == nil"];

“==”是逻辑上的相等。只是“=”是一项任务。

我一直犯这个错误。

编辑：

好吧，我不知何故错过了他说他已经尝试过的OP。对不起。

于 2010-04-30T17:47:29.380 回答