如何使用NSPredicate从CoreData获取对象

Question

我正在使用CoreData来存储一些实体。我在CoreData中有一些实体，想要使用NSPredicate来获取它们。我使用以下代码但返回的objectsArray为空。谁能告诉我我做错了什么？

AppDelegate *delegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];  
NSManagedObjectContext *context = delegate.managedObjectContext;  
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Devices"];  
NSError *error;  

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"deviceName == %@",  deviceNameString];  
[request setPredicate:predicate];  
[request setEntity:[NSEntityDescription entityForName:@"Devices" inManagedObjectContext: context]];  

NSArray *objectsArray = [context executeFetchRequest:request error:&error];