我是CoreData的新手，我试图使用 NSFetchedResultsController 来获取它 . 在初始化 NSFetchedResultsController 时，我遇到了这个运行时异常：
2018-12-09 15:03:20.493509+0800 [5184:148001] [error] error:
No NSEntityDescriptions in any model claim the NSManagedObject subclass
'Product' so +entity is confused. Have you loaded your
NSManagedObjectModel yet ?
CoreData: error: No NSEntityDescriptions in any model claim the
NSManagedObject subclass 'Product' so +entity is confused. Have you
loaded your NSManagedObjectModel yet ?
2018-12-09 15:03:20.493718+0800[5184:148001] [error] error: +
[Product entity] Failed to find a unique match for an
NSEntityDescription to a managed object subclass
CoreData: error: +[Product entity] Failed to find a unique match for an
NSEntityDescription to a managed object subclass
可能是什么原因？