Method to quickly perform a block using a temporary managed object context. Can be useful if you need to perform a few pre-migration optimisations to a database.

NSManagedObject+temporaryContext.m

typedef NSError *(^NSManagedObjectContextBlock)(NSManagedObjectContext *context);

+ (void)performBlock:(NSManagedObjectContextBlock)block withTemporaryManagedObjectContextFromDatabaseAtURL:(NSURL *)url withModelBundle:(NSBundle *)bundle
{
    NSParameterAssert(block);
    if (!block)
    {
        return;
    }
    
    NSDictionary *storeMetadata = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:NSSQLiteStoreType URL:url error:nil];
    NSManagedObjectModel *sourceModel = [NSManagedObjectModel mergedModelFromBundles:@[bundle] forStoreMetadata:storeMetadata];
    if (sourceModel) // If sourceModel cannot be found, cannot create a context
    {
        @autoreleasepool {
            // Create a temporary persistentStoreCoordinator based on source model
            NSPersistentStoreCoordinator *temporaryStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:sourceModel];
            NSError *error;
            [temporaryStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
                                                    configuration:nil
                                                              URL:url
                                                          options:@{NSIgnorePersistentStoreVersioningOption:@YES}
                                                            error:&error];
            if (!error)
            {
                NSManagedObjectContext *temporaryManagedObjectContext = [[NSManagedObjectContext alloc] initWithConcurrencyType: NSMainQueueConcurrencyType];
                temporaryManagedObjectContext.persistentStoreCoordinator = temporaryStoreCoordinator;
                
                block(temporaryManagedObjectContext);
                
                temporaryManagedObjectContext = nil; // Clean up to release any ownership of DB store
            }
            temporaryStoreCoordinator = nil; // clean up to release any ownership of DB store
        }
    }
}