ios - 如何从核心数据表 ios 中检索数据？

Question

嗨，我在我的测试项目中使用了核心数据。我能够将 json 中的内容添加到核心数据实体“书籍”。但是当我尝试检索数据时，我只得到最后一个“数据”(总共 37 个数据)？我创建了一个可变数组“directors”来插入每个检索到的值，但这也不起作用。

请检查我的代码。

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


    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"Book" inManagedObjectContext:context];
    [fetchRequest setEntity:entity];
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];

    for (NSManagedObject *info in fetchedObjects) {
        NSLog(@"1");
        [directors addObject:[info valueForKey:@"director"]];
        NSLog(@"Name: %@", info);

    }
    NSLog(@"%@",directors);

正在打印'NSLog(@"1"); 来检查循环工作了多少次，但是并且得到这样的输出

2014-06-15 15:40:03.665 test[12489:1303] Name: <Book: 0x8c50580> (entity: Book; id: 0x8c66a60 <x-coredata://EEF91D97-C982-40E1-A898-9E646D206B39/Book/p1> ; data: <fault>)
2014-06-15 15:40:03.665 test[12489:1303] 1
2014-06-15 15:40:03.666 test[12489:1303] Name: <Book: 0x8c690d0> (entity: Book; id: 0x8c68420 <x-coredata://EEF91D97-C982-40E1-A898-9E646D206B39/Book/p2> ; data: <fault>)
2014-06-15 15:40:03.666 test[12489:1303] 1
2014-06-15 15:40:03.667 test[12489:1303] Name: <Book: 0x8f9c8d0> (entity: Book; id: 0x8d89870 <x-coredata://EEF91D97-C982-40E1-A898-9E646D206B39/Book/p3> ; data: {
    actors = "Hazel Crowney, Kiran Kumar, Shahbaaz Khan, Usha Bachani, Mohammed Iqbal Khan, Alka Verma";
    censor = U;
    director = "Arshad Yusuf Pathan";
    global = N;
    lang = Hindi;
    length = "";
    rating = 0;
    releasedate = "13th Jun 2014";
    synopsis = "The story of a racecar driver who loses his eyesight. While he suffers a setback, he learns about the true value of the relationships in his life. Through the course of the film, the hero realizes the";
    title = Unforgettable;
    trailer = "";
    type = MT;
    url = "http://cnt.in.bookmyshow.com/Events/Mobile/ET00022474.jpg?dtm=15614303";
})

当前代码

Answer 1

正如我从日志中看到的那样，除了最后一个之外，您从 CoreData 收到的所有托管对象都是错误的。这意味着这些对象是表示实体(书籍)的适当类的实例，但它们未被初始化。如果您尝试访问接收记录的某些字段，它们将自动初始化。我想对您的代码进行以下修改应该可行:

for (Book *info in fetchedObjects) {
    NSLog(@"1");
    [directors addObject:info.director];
    NSLog(@"Name: %@", info);

}
NSLog(@"%@",directors);

这里的Book是一个继承自NSManagedObject的类，表示名为Book的实体。您可以通过突出显示 CoreData 模型中的实体并从 Editor 菜单中选择 Create NSManagedObject subclass 项来为任何实体生成此类。

有关故障的更多信息:Core Data Programming Gudie