应用程序内存不足，在大型数据存储上执行数据迁移。我怎么能在多次通过中做到这一点？

Question

当我的应用尝试使用标准迁移将旧核心数据迁移到新商店时，如果核心数据存储很大，那么内存耗尽并崩溃。

我已经看到它在几个地方提到解决方案是在'多次通过'中做到这一点，但苹果文档在解释这个问题上并不太好：

http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/CoreDataVersioning/Articles/vmCustomizing.html#//apple_ref/doc/uid/TP40004399-CH8-SW9

这里的教程要好得多，显示代码，但是我对此代码的位置感到困惑：

Example or explanation of Core Data Migration with multiple passes?

这是我的代码：

- (NSManagedObjectContext *)managedObjectContext {

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

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (coordinator != nil) {
        managedObjectContext = [NSManagedObjectContext new];
        [managedObjectContext setPersistentStoreCoordinator: coordinator];
    }
    return managedObjectContext;
}

- (NSManagedObjectModel *)managedObjectModel {

    if (managedObjectModel != nil) {
        return managedObjectModel;
    }
    //managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];
    NSString *modelPath = [[NSBundle mainBundle] pathForResource:@"EntryDatabase" ofType:@"momd"];
    NSURL *modelURL = [NSURL fileURLWithPath:modelPath];
    managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];

    return managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {

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

    NSString *storePath = [[self applicationDocumentsDirectory] stringByAppendingPathComponent:@"CoreDataStore.sqlite"];

    NSFileManager *fileManager = [NSFileManager defaultManager];

    // If the expected store doesn't exist, copy the default store.
    if (![fileManager fileExistsAtPath:storePath]) {

        NSString *defaultStorePath = [[NSBundle mainBundle] pathForResource:@"CoreDataStore" ofType:@"sqlite"];
        if (defaultStorePath) {
            NSError *error;
            [fileManager copyItemAtPath:defaultStorePath toPath:storePath error:&error];
            NSLog(@"Error");
        }
    }

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

    NSURL *storeUrl = [NSURL fileURLWithPath:storePath];
    NSError *error;
    NSDictionary *pscOptions = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                                [NSNumber numberWithBool:NO], NSInferMappingModelAutomaticallyOption,
                                nil];

    if (![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:pscOptions error:&error]) {

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

    return persistentStoreCoordinator;
}

我不确定如何在我的代码中实现多次传递 - 例如应该去哪里。