Undefined symbols for architecture error

I am using Core Data to show data in a table view, but I constantly encounter this problem : Undefined symbols for architecture i386: "_OBJC_CLASS_$_NSEntityDescription", referenced from: objc-class-ref in singleAppDelegate.o objc-class-ref in singleViewController.o "_OBJC_CLASS_$_NSFetchRequest", referenced from: objc-class-ref in singleViewController.o "_OBJC_CLASS_$_NSFetchedResultsController", referenced from: objc-class-ref in singleViewController.o "_OBJC_METACLASS_$_NSManagedObject", referenced from: _OBJC_METACLASS_$_Data in Data.o "_OBJC_CLASS_$_NSManagedObject", referenced from: _OBJC_CLASS_$_Data in Data.o here is my singleViewController.m ``` #import "Data.h" #import "singleViewController.h" @interface singleViewController () @end @implementation singleViewController @synthesize managedObjectContext; @synthesize fetchedResultsController = _fetchedResultsController; - (id)initWithStyle:(UITableViewStyle)style { self = [super initWithStyle:style]; if (self) { // Custom initialization } return self; } - (void)viewDidLoad { [super viewDidLoad]; NSError *error; if (![[self fetchedResultsController] performFetch:&error]) { // Update to handle the error appropriately. NSLog(@"Unresolved error %@, %@", error, [error userInfo]); exit(-1); // Fail } self.title = @"Core Data"; } - (void)viewDidUnload { [super viewDidUnload]; self.fetchedResultsController = nil; } - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation { return (interfaceOrientation == UIInterfaceOrientationPortrait); } #pragma mark - Table view data source - (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView { // Return the number of sections. return 1; } - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section { // Return the number of rows in the section. id sectionInfo = [[_fetchedResultsController sections] objectAtIndex:section]; return [sectionInfo numberOfObjects]; } - (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath { Data *info = [_fetchedResultsController objectAtIndexPath:indexPath]; cell.textLabel.text = info.title; cell.detailTextLabel.text = info.content; } - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { static NSString *CellIdentifier = @"Cell"; UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier]; if (!cell) { cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier]; } // Configure the cell... [self configureCell:cell atIndexPath:indexPath]; return cell; } #pragma mark - fetchedResultsController - (NSFetchedResultsController *)fetchedResultsController { if (_fetchedResultsController != nil) { return _fetchedResultsController; } NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Data" inManagedObjectContext:managedObjectContext]; [fetchRequest setEntity:entity]; [fetchRequest setFetchBatchSize:20]; NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"]; self.fetchedResultsController = theFetchedResultsController; _fetchedResultsController.delegate = self; return _fetchedResultsController; } - (void)controllerWillChangeContent:(NSFetchedResultsController *)controller { // The fetch controller is about to start sending change notifications, so prepare the table view for updates. [self.tableView beginUpdates]; } - (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath { UITableView *tableView = self.tableView; switch(type) { case NSFetchedResultsChangeInsert: [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade]; break; case NSFetchedResultsChangeDelete: [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade]; break; case NSFetchedResultsChangeUpdate: [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath]; break; case NSFetchedResultsChangeMove: [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade]; [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade]; break; } } - (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id )sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type { switch(type) { case NSFetchedResultsChangeInsert: [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade]; break; case NSFetchedResultsChangeDelete: [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade]; break; } } - (void)controllerDidChangeContent:(NSFetchedResultsController *)controller { // The fetch controller has sent all current change notifications, so tell the table view to process all updates. [self.tableView endUpdates]; } @end ``` and here is my appdelegate.m ``` #import "Data.h" #import "singleAppDelegate.h" #import "singleAppDelegate.h" @implementation singleAppDelegate @synthesize window = _window; @synthesize managedObjectContext = __managedObjectContext; @synthesize managedObjectModel = __managedObjectModel; @synthesize persistentStoreCoordinator = __persistentStoreCoordinator; - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { NSManagedObjectContext *context = [self managedObjectContext]; Data *data = [NSEntityDescription insertNewObjectForEntityForName:@"Data" inManagedObjectContext:context]; data.title = @"title"; data.content = @"test content"; NSError *error; if (![context save:&error]) { NSLog(@"Whoops, couldn't save: %@", [error localizedDescription]); } return YES; } ``` and here is my data.m ``` @implementation Data @dynamic title; @dynamic content; @end ``` Don't know what's wrong.