Core Data migrations are hard. If you don't need to migrate, then don't! Here's how:

gistfile1.m

/*
Core Data is great but automatic migrations can be tricky. Migrations can take a long time, which could
result in [your app being terminated](http://stackoverflow.com/questions/13333289/core-data-timeout-adding-persistent-store-on-application-launch) 
if it is happening on the main thread during application launch. Performing migrations on a background 
thread is also a [bad idea](http://stackoverflow.com/a/2866725/503916), meaning your application really
needs to be able to fully launch *without a Core Data stack whatsoever* in order to safely migrate. 
This can be a huge change to make to an existing app.

If you're really only using Core Data as a cache, you don't actually *need* to perform a migration. 
Simply check if the existing store is compatible with your managed object model and if so, delete 
and recreate it. Here's my approach for doing so. Many thanks to Marcus Zarra for being *extremely* 
helpful both over Twitter and on Stack Overflow.
*/

NSString *persistentStorePath = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, 
                                 YES) lastObject] stringByAppendingPathComponent:@"AppName.sqlite"];

// Create managed object model

NSManagedObjectModel *managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:
                                           [[NSBundle mainBundle] URLForResource:@"AppName" 
                                                                   withExtension:@"momd"]];

// Create persistent store coordinator

NSPersistentStoreCoordinator *persistentStoreCoordinator = 
    [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:managedObjectModel];

// Create managed object context

managedObjectContext = [[NSManagedObjectContext alloc] init];
managedObjectContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy;
managedObjectContext.persistentStoreCoordinator = persistentStoreCoordinator;

NSURL *persistentStoreURL = [NSURL fileURLWithPath:persistentStorePath];

if ([[NSFileManager defaultManager] fileExistsAtPath:persistentStorePath]) {
    // If store already exists, see if it is compatible with managed object model
    
    NSError *storeMetadataError = nil;
    NSDictionary *storeMetadata = 
            [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:NSSQLiteStoreType
                                                                       URL:persistentStoreURL
                                                                     error:&storeMetadataError];
    
    if (storeMetadataError || ![managedObjectModel isConfiguration:nil 
                                       compatibleWithStoreMetadata:storeMetadata]) {
        // Store is not compatible - wipe it
        
        NSError *removeStoreError = nil;
        [[NSFileManager defaultManager] removeItemAtPath:persistentStorePath error:&removeStoreError];
        
        if (removeStoreError)
            NSLog(@"Error removing file at path '%@': %@, %@", path, removeStoreError, 
                  [removeStoreError userInfo]);
    }
}

NSError *addStoreError = nil;

if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil
                                                        URL:persistentStoreURL options:nil
                                                      error:&addStoreError])
    NSLog(@"Unable to add store: %@, %@", addStoreError, [addStoreError userInfo]);

The code looks good to me. Removing the instance level ivars removes the risk of someone overriding an accessor and doing something silly.

Thanks!