托管对象值随机为null(包括示例项目)

Question

将数据(10条记录)保存到实体后,我正在处理获取请求以再次获取所有数据:

//Saving data
- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName{

    //Save to coredata


        song = [NSEntityDescription insertNewObjectForEntityForName:@"Song"
                                             inManagedObjectContext:context];
        [song setValue:title forKey:@"title"];

        [song setValue:songLink forKey:@"songWebLink"];
        NSLog(@"Song link : %@",songLink);//Never get NULL


        [song setValue:albumLink forKey:@"albumImageLink"];

        NSError *error = nil;
        if (![context save:&error]) {
            NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]);
        }else{

            NSLog(@"Record saved correctly");
        }
}

保存上面的工作正常,我在保存到上下文之前非常仔细地调试了所有数据,以确保没有任何属性NULL.

问题始终在于songWebLink属性,有时当我尝试将其返回到下面时它会变为null:

- (void)parserDidEndDocument:(NSXMLParser *)parser{

    NSEntityDescription *songEntity=[NSEntityDescription entityForName:@"Song" inManagedObjectContext:context];
    NSFetchRequest *fetch=[[NSFetchRequest alloc] init];
    [fetch setEntity:songEntity];
    NSError *fetchError;
    NSArray *fetchedSongs=[context executeFetchRequest:fetch error:&fetchError];

    NSMutableArray *songs = [[NSMutableArray alloc] init];

    for (NSManagedObject *songObject in fetchedSongs) {
    //here is the issue: this for loop will go through 10 iterations, songWebLink is randomly NULL, sometimes it's the fourth iteration, sometimes the 8th, sometimes the 5th.
        NSLog(@"song web link: %@",[songObject valueForKey:@"songWebLink"]);//albumImageLink and title attributes are fine
    }
}

问题是,当我NSLog的songWebLink,它得到NULL,一旦第4次迭代,然后6日,然后第二等,这是随机读取时分配NULL一个属性.保存数据时,我确保没有NULL值songWebLink.所以我打赌别的东西是造成这个问题的原因.

有什么想法吗？

编辑

以下是MOC的初始化方式:

AppDelegate.m:

- (NSManagedObjectContext *)managedObjectContext
{
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        _managedObjectContext = [[NSManagedObjectContext alloc] init];//I tried initWithConcurrencyType:NSMainQueueConcurrencyType
        [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    }
    return _managedObjectContext;
}

获取MOC对象以在其他类中使用它:

AppDelegate *appDelegate = (AppDelegate*)[[UIApplication sharedApplication]delegate];
context = [appDelegate managedObjectContext];

示例项目: 如果您认为需要查看应用程序项目,我制作了一个简化版本,我在其上复制了该错误,您可以在此处下载.

Answer 1

经过几天的尝试解决这个问题后，我选择了MagicalRecord，因为它简化了许多令人头疼的问题。如果有人能解决这个问题并帮助我，我会认为他的答案是公认的。