用NSFetchRequest获取NSArray，里面都是托管对象。如果上下文里没有数据，就会从持久化存储区里获取。
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Item"];
NSArray *itemObjects = [_coreDataHelper.context executeFetchRequest:request error:nil];
for (Item *item in itemObjects) {
NSLog(@"item name = %@", item.name);
}