循环遍历NSFetchRequest结果

Question

我对Core Data和Objective-C整体来说都是新手，所以我希望我的措辞是正确的，我已经能够使用Core Data存储和检索数据，但我正在努力遍历结果集。

// read the data
NSFetchRequest *req = [[NSFetchRequest alloc]init];
[req setEntity:[NSEntityDescription entityForName:@"Card" inManagedObjectContext:app.managedObjectContext]];
//[req setPredicate:[NSPredicate predicateWithFormat:@"cardID == %@", @"1"]];
NSArray *crd = [[app.managedObjectContext executeFetchRequest:req error:nil] lastObject];

for(Card *card in crd){
    NSLog(@"%@",card.backImageURL);
}

当我尝试运行这个应用程序时，我一直收到这个错误，

2012-10-16 11:37:29.407 Core test[925:17903] -[Card countByEnumeratingWithState:objects:count:]: unrecognized selector sent to instance 0x962f5c0
2012-10-16 11:37:29.408 Core test[925:17903] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Card countByEnumeratingWithState:objects:count:]: unrecognized selector sent to instance 0x962f5c0'

我确信这是非常简单的事情，但由于某些原因，我似乎不能准确地指出问题所在。

Answer 1

而不是

NSArray *crd = [[app.managedObjectContext executeFetchRequest:req error:nil] lastObject];

它应该是

NSArray *crd = [app.managedObjectContext executeFetchRequest:req error:nil];

如果出现错误，则应使用error参数-

NSError *error = nil;
NSArray *crd = [app.managedObjectContext executeFetchRequest:req error:&error];
if(error){
    NSLog(@"ERROR: %@", [error localizedDescription]);
}