无法转换类型的值＆＃39;任务＆＃39;预期的论证类型＆＃39;课程作业＆＃39;

Question

好的，我的fetch结果控制器出了问题。问题似乎是，当要求它从另一个表中提取数据时，异常发生在它似乎试图将一个表转换为另一个表的情况下。这是为什么？

var fetchedResultsController: NSFetchedResultsController<Task> {
        if _fetchedResultsController != nil {
            return _fetchedResultsController! as! 
NSFetchedResultsController<Task>
        }

        let fetchRequest: NSFetchRequest<Task> = Task.fetchRequest()

        // Set the batch size to a suitable number.
        fetchRequest.fetchBatchSize = 20

        // Edit the sort key as appropriate.
        let sortDescriptor = NSSortDescriptor(key: "coursework", ascending: 
true)

        fetchRequest.sortDescriptors = [sortDescriptor]

        let currentCoursework = self.detailItem
        if(self.detailItem != nil){
            let predicate = NSPredicate(format: "moduleCoursework", currentCoursework!)
            fetchRequest.predicate = predicate
        }

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
        let aaFetchedResultsController = 
NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: 
self.managedObjectContext, 
sectionNameKeyPath:#keyPath(Coursework.modulename), cacheName: "Master")
        aaFetchedResultsController.delegate = self
        _fetchedResultsController = aaFetchedResultsController

        do {
            try _fetchedResultsController!.performFetch()
        } catch {
        /* Replace this implementation with code to handle the error 
appropriately.
         fatalError() causes the application to generate a crash log and 
terminate. You should not use this function in a shipping application, 
although it may be useful during development.*/
            let nserror = error as NSError
            fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
        }

        return _fetchedResultsController! as! 
NSFetchedResultsController<Task>
    }
    var _fetchedResultsController: NSFetchedResultsController<Coursework>? = 
nil