Load a previous model version

Question

I am loading a NSManagedObjectModel model with the initWithContentsOfURL: constructor like this:

NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"MyDocument" withExtension:@"momd"];
NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];

However this only gives me access to the latest/current version of a model. Is it posible to load previous versions with the same momd file? how?

Answer 1

Actually you can load an older version with:

- (NSManagedObjectModel *)managedObjectModelForVersion:(NSString *)version
{
        NSURL *modelURL = [[NSBundle mainBundle] URLForResource:[NSString stringWithFormat:@"AppModel.momd/AppModel %@",version] withExtension:@"mom"];
        NSManagedObjectModel *model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
        return model;
 }

Just replace AppModel with your model name.

I'm using this to get myself out of a sticky manual migration situation involving iCloud. Searched high and low and couldn't find this anywhere.

Answer 2

If you just want to load the version of the model that's compatible with a particular existing store try:

NSError *error = nil;
NSDictionary *storeMetadata = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:NSSQLiteStoreType 
                                                                                         URL:storeURL 
                                                                                       error:&error];
NSManagedObjectModel *oldManagedObjectModel = [NSManagedObjectModel mergedModelFromBundles:[NSArray arrayWithObject:[NSBundle mainBundle]] 
                                                                          forStoreMetadata:storeMetadata];

Note that if you use XCode version identifiers for your data model versions, the persistent store's current version identifiers are accessible through the NSStoreModelVersionIdentifiersKey entry in the store metadata dictionary.

As far as loading a particular arbitrary version is concerned, the mom files are typically located under the momd directory in your app's bundle, so you could enumerate them using NSFileManager. I believe to find one with a particular version identifier you would have to use NSManagedObjectModel's initWithContentsOfURL: initializer and then inspect the versionIdentifiers property, or use the isConfiguration:compatibleWithStoreMetadata: instance method to determine compatibility.