'+entityForName: nil is not a legal NSManagedObjectContext

Question

I am getting an error when I am trying to load my data from Core Data to a Table View controller. I believe this is due to the managedObject not being passed down correctly.

But I believe I did so after some research with the code in the AppDelegate.m under didFinishLaunchingWithOptions.

error

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+entityForName: nil is not a legal NSManagedObjectContext parameter searching for entity name 'Club''

MLVAPPDelegate.h

#import <UIKit/UIKit.h>

@interface MLVAppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (readonly, strong, nonatomic) NSManagedObjectContext *managedObjectContext;
@property (readonly, strong, nonatomic) NSManagedObjectModel *managedObjectModel;
@property (readonly, strong, nonatomic) NSPersistentStoreCoordinator *persistentStoreCoordinator;

@end

MLVAppDelegate.m

#import "MLVAppDelegate.h"
#import "MLVSyncEngine.h"
#import "Club.h"
#import "IronSet.h"
#import "MLVClubSelectionViewController.h"


@implementation MLVAppDelegate

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    UITabBarController *tabController = (UITabBarController *)self.window.rootViewController;

    UINavigationController *navigationController = (UINavigationController *)[[tabController viewControllers] objectAtIndex:0];
    MLVClubSelectionViewController *controller = (MLVClubSelectionViewController *)[[navigationController viewControllers] objectAtIndex:0];
    controller.managedObjectContext = self.managedObjectContext;

    [[MLVSyncEngine sharedEngine] registerNSManagedObjectClassToSync:[Club class]];

    [[MLVSyncEngine sharedEngine] registerNSManagedObjectClassToSync:[IronSet class]];

    return YES;
}

- (void)applicationDidBecomeActive:(UIApplication *)application
{
    [[MLVSyncEngine sharedEngine] startSync];
}

- (NSManagedObjectContext *)masterManagedObjectContext {
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        _managedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
        [_managedObjectContext performBlockAndWait:^{
            [_managedObjectContext setPersistentStoreCoordinator:coordinator];
        }];
    }
    return _managedObjectContext;
}
- (NSManagedObjectModel *)managedObjectModel
{
    if (_managedObjectModel != nil) {
        return _managedObjectModel;
    }
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"GolfCalculator" withExtension:@"momd"];
    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    return _managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }

    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"GolfCalculator.sqlite"];

    NSError *error = nil;
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {

        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _persistentStoreCoordinator;
}

#pragma mark - Application's Documents directory

// Returns the URL to the application's Documents directory.
- (NSURL *)applicationDocumentsDirectory
{
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

@end

MLVClubSelectionViewController.h

#import <UIKit/UIKit.h>

@interface MLVClubSelectionViewController : UITableViewController

@property (nonatomic, strong) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, retain) NSFetchedResultsController *fetchedResultsController;

@end

MLVClubSelectionViewController.m

#import "MLVClubSelectionViewController.h"
#import "Club.h"
#import "IronSet.h"
#import "MLVCoreDataController.h"

@interface MLVClubSelectionViewController ()

@end

@implementation MLVClubSelectionViewController
@synthesize managedObjectContext;
@synthesize fetchedResultsController = _fetchedResultsController;

- (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 = @"Club Selection";
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
#warning Potentially incomplete method implementation.
    // Return the number of sections.
    return 0;
}

- (NSInteger)tableView:(UITableView *)tableView
 numberOfRowsInSection:(NSInteger)section {
    id  sectionInfo =
    [[_fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath {
    Club *info = [_fetchedResultsController objectAtIndexPath:indexPath];
    cell.textLabel.text = info.fullTitle;
    cell.detailTextLabel.text = [NSString stringWithFormat:@"%@, %@",
                                 info.model, info.level];
}

- (UITableViewCell *)tableView:(UITableView *)tableView
         cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell =
    [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    // Set up the cell...
    [self configureCell:cell atIndexPath:indexPath];

    return cell;
}


#pragma mark - Table view delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{

}

#pragma mark - Fetched Results 
- (NSFetchedResultsController *)fetchedResultsController {

    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription
                                   entityForName:@"Club" inManagedObjectContext:managedObjectContext];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sort = [[NSSortDescriptor alloc]
                              initWithKey:@"make" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];

    [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];
}
- (void)viewDidUnload {
    self.fetchedResultsController = nil;
}

@end

Answer 1

You have implemented the method masterManagedObjectContext, yet you never call it. You are instead assigning the value of the managedObjectContext property (which, as all object properties, will be initialized to nil), as the context passed to your controller.

You could do one of the following:

1. Rename masterManagedObjectContext to managedObjectContext, which will provide a lazily-loaded implementation of the managedObjectContext accessor (meaning that self.managedObjectContext should no longer evaluate to nil)

2. You could alternatively implement managedObjectContext as follows:

   - (NSManagedObjectContext *)managedObjectContext {
       return [self masterManagedObjectContext];
   }
   

3. Remove the managedObjectContext property from the app delegate and only use the masterManagedObjectContext method. This will also require explicitly defining the _managedObjectContext instance variable.