通过NSFetchRequest循环结果

Question

我对Core Data很新，实际上Objective-C是一个整体，所以我希望我正确地表述了这一点，我已经能够使用Core Data存储和检索数据，但我挣扎着通过结果集循环，

// 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); 
} 

我不断收到这个错误，虽然当我尝试运行应用程序，

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' 

我相信这是一些非常简单的，但由于某种原因，我可以”似乎查明问题。

Answer 1

而不是

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

应该

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

如果您收到错误，那么你应该使用错误的参数 -

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