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当我的应用程序尝试使用标准迁移将旧核心数据迁移到新存储时，如果核心数据存储很大，那么它会耗尽内存并崩溃。

我已经在几个地方看到它提到解决方案是在“多次传递”中进行，但苹果文档在解释这一点时并不太好：

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

这里的本教程要好得多，显示了代码，但我对这段代码的去向感到困惑：

多遍核心数据迁移的示例或解释？

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;
}

我不确定如何在我拥有的代码中实现多次传递 - 例如它应该去哪里。

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