Reset a CoreData persistent store

Question

Hi everybody,

Basically, what I'm trying to do is to wipe out all data in my CoreData persistent store, then import new data. How would you do this? It seems that the simplest solution is call [NSPersistentStoreCoordinator removePersistentStore:error:] and then remove the file. Is that the best practice available? Is it thread-safe?

Thank you very much,

#

Question 0.1: was

I am trying to update the data in a CoreData persistent store. My user is seeing a table view with statistical data. I want to update the application by deleting all existing data, then importing new data. I would like to show a progress view to tell the user that the application is not hanging.

I have added the following resetPersistentStore method in my AppDelegate (persistentStoreCoordinatoris given for reference):

// ...
@property (nonatomic, retain, readonly) NSManagedObjectModel *managedObjectModel;
@property (nonatomic, retain, readonly) NSManagedObjectContext *managedObjectContext;
@property (nonatomic, retain, readonly) NSPersistentStoreCoordinator *persistentStoreCoordinator;
// ...

/**
 Returns the persistent store coordinator for the application.
 If the coordinator doesn't already exist, it is created and the application's store added to it.
 */
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

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

    NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory] stringByAppendingPathComponent: kPersistentStoreFilename]];

 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;
}

/**
 * Will remove the persistent store
 */
- (NSPersistentStoreCoordinator *)resetPersistentStore {
 NSError *error;

 [managedObjectContext lock];

 // FIXME: dirty. If there are many stores...
 NSPersistentStore *store = [[persistentStoreCoordinator persistentStores] objectAtIndex:0];

 if (![persistentStoreCoordinator removePersistentStore:store error:&error]) {
  NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
  abort();
    }  

 // Delete file
 if (![[NSFileManager defaultManager] removeItemAtPath:store.URL.path error:&error]) {
  NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
  abort();
    } 

 // Delete the reference to non-existing store
 [persistentStoreCoordinator release];
 persistentStoreCoordinator = nil;

 NSPersistentStoreCoordinator *r = [self persistentStoreCoordinator];
 [managedObjectContext unlock];

 return r;
}

Then in my view I do (in another thread since I am using MBProgressHUD:

PatrimoineAppDelegate *appDelegate = (PatrimoineAppDelegate *)[[UIApplication sharedApplication] delegate]; 
// Delete everything
[appDelegate resetPersistentStore];

And I get an EXC_BAD_ACESS...

I do not know CoreData or multithreading very well, maybe I'm doing an evident error...

Answer 1

If your goal is to empty the data store and reload it with new information, you may be better off using NSManagedObjectContext's reset and then loading in new data.

From NSManagedObjectContext's documentation

A context always has a “parent” persistent store coordinator which provides the model and dispatches requests to the various persistent stores containing the data. Without a coordinator, a context is not fully functional. The context’s coordinator provides the managed object model and handles persistency. All objects fetched from an external store are registered in a context together with a global identifier (an instance of NSManagedObjectID) that’s used to uniquely identify each object to the external store.

Removing the persistent store and using the managed object context associated with the store is probably the cause of the error.

Answer 2

Here is the solution. There may be some more elegant options (lock...) but this one works.

/**
 * Will remove the persistent store
 */
- (NSPersistentStoreCoordinator *)resetPersistentStore {
    NSError *error = nil;

    if ([persistentStoreCoordinator persistentStores] == nil)
        return [self persistentStoreCoordinator];

    [managedObjectContext reset];
    [managedObjectContext lock];

    // FIXME: dirty. If there are many stores...
    NSPersistentStore *store = [[persistentStoreCoordinator persistentStores] lastObject];

    if (![persistentStoreCoordinator removePersistentStore:store error:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }  

    // Delete file
    if ([[NSFileManager defaultManager] fileExistsAtPath:store.URL.path]) {
        if (![[NSFileManager defaultManager] removeItemAtPath:store.URL.path error:&error]) {
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        } 
    }

    // Delete the reference to non-existing store
    [persistentStoreCoordinator release];
    persistentStoreCoordinator = nil;

    NSPersistentStoreCoordinator *r = [self persistentStoreCoordinator];
    [managedObjectContext unlock];

    return r;
}