核心数据自动轻迁移问题

Question

我在应用程序委托中使用自动轻量级迁移代码时遇到问题！

我阅读了所有关于 "自动轻量级迁移" 但毕竟我找不到使用代码的方法准备自动轻量级迁移。

最近，我刚刚向我的数据模型中的Entity添加了一些新的Attribute，并且我想保留我的旧数据。

我的应用程序委托代码是这样的：

- (NSPersistentStoreCoordinator *) persistentStoreCoordinator {


    if (__persistentStoreCoordinator) {
        return __persistentStoreCoordinator;
    }

    NSManagedObjectModel *mom = [self managedObjectModel];
    if (!mom) {
        NSLog(@"%@:%@ No model to generate a store from", [self class], NSStringFromSelector(_cmd));
        return nil;
    }

    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSURL *applicationFilesDirectory = [self applicationFilesDirectory];
    NSError *error = nil;

    NSDictionary *properties = [applicationFilesDirectory resourceValuesForKeys:[NSArray arrayWithObject:NSURLIsDirectoryKey] error:&error];

    if (!properties) {
        BOOL ok = NO;
        if ([error code] == NSFileReadNoSuchFileError) {
            ok = [fileManager createDirectoryAtPath:[applicationFilesDirectory path] withIntermediateDirectories:YES attributes:nil error:&error];
        }
        if (!ok) {
            [[NSApplication sharedApplication] presentError:error];
            return nil;
        }
    }
    else {
        if ([[properties objectForKey:NSURLIsDirectoryKey] boolValue] != YES) {
            // Customize and localize this error.
            NSString *failureDescription = [NSString stringWithFormat:@"Expected a folder to store application data, found a file (%@).", [applicationFilesDirectory path]]; 

            NSMutableDictionary *dict = [NSMutableDictionary dictionary];
            [dict setValue:failureDescription forKey:NSLocalizedDescriptionKey];
            error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code:101 userInfo:dict];

            [[NSApplication sharedApplication] presentError:error];
            return nil;
        }
    }

    NSURL *url = [applicationFilesDirectory URLByAppendingPathComponent:@"FinancingPro.storedata"];
    __persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:mom];
    if (![__persistentStoreCoordinator addPersistentStoreWithType:NSXMLStoreType configuration:nil URL:url options:nil error:&error]) {
        [[NSApplication sharedApplication] presentError:error];
        [__persistentStoreCoordinator release], __persistentStoreCoordinator = nil;
        return nil;
    }

    return __persistentStoreCoordinator;

现在我不知道如何更改此代码以实现自动轻量级迁移！
请注意，我的数据库不是 SQLlite。

Answer 1

您需要在此处使用 NSInferMappingModelAutomaticallyOption 键设置选项字典：

if (![__persistentStoreCoordinator addPersistentStoreWithType:NSXMLStoreType configuration:nil URL:url options:nil error:&error]) {

通过为 options 传递 nil 值，您将告诉存储忽略任何迁移。