1

请帮忙！我一直试图弄清楚这一点太久了。

我似乎无法在 fetchedResultsController 方法中使用 NSPredicate：

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

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Tweet" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"column == 0"];
[fetchRequest setPredicate:predicate];

[fetchRequest setReturnsObjectsAsFaults:NO];
[fetchRequest setFetchBatchSize:20];

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

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;

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

return fetchedResultsController; }

然而，在另一种方法中，我只是检查一个对象是否存在，谓词就像一个魅力：

- (BOOL)findObjectWithKey:(NSString *)key andValue:(NSString *)value sortBy:(NSString *)sort {
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Tweet" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

NSPredicate *predicate = [NSPredicate predicateWithFormat: @"%K == %@", key, value];
[request setPredicate:predicate];

[request setReturnsObjectsAsFaults:NO];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:sort ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
aFetchedResultsController.delegate = self;

NSError *error = nil;
NSArray *result = [managedObjectContext executeFetchRequest:request error:&error];

[aFetchedResultsController release];
[request release];
[sortDescriptor release];
[sortDescriptors release];

if ((result != nil) && ([result count]) && (error == nil)) {
    return TRUE;
} else {
    return FALSE;
}}

我究竟做错了什么？提前致谢！

4

2 回答 2

0

由于列是实体上的属性，因此您的谓词逻辑很好。您看到哪些结果表明谓词不起作用？

于 2010-03-16T23:53:56.983 回答
0

假设您的“列”属性是一个数字，您应该将其与 NSNumber 对象进行比较，如下所示：

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"column == %@",[NSNumber numberWithInteger:0]];
于 2010-03-16T21:46:51.280 回答