Core Data migration problem: "Persistent store migration failed, missing source managed object model."

Question

The Background

• A Cocoa Non Document Core Data project with two Managed Object Models.
• Model 1 stays the same. Model 2 has changed, so I want to migrate the store.
• I've created a new version by Design > Data Model > Add Model Version in Xcode.
• The difference between versions is a single relationship that's been changed from to a one to many.
• I've made my changes to the model, then saved.
• I've made a new Mapping Model that has the old model as a source and new model as a destination.
• I've ensured all Mapping Models and Data Models and are being compiled and all are copied to the Resource folder of my app bundle.
• I've switched on migrations by passing in a dictionary with the NSMigratePersistentStoresAutomaticallyOption key as [NSNumber numberWithBool:YES] when adding the Persistent Store.
• Rather than merging all models in the bundle, I've specified the two models I want to use (model 1 and the new version of model 2) and merged them using modelByMergingModels:

The Problem

No matter what I do to migrate, I get the error message:

"Persistent store migration failed, missing source managed object model."

What I've Tried

• I clean after every single build.
• I've tried various combinations of having only the model I'm migrating to in Resources, being compiled, or both.
• Since the error message implies it can't find the source model for my migration, I've tried having every version of the model in both the Resources folder and being compiled.
• I've made sure I'm not making a really basic error by switching back to the original version of my data model. The app runs fine.
• I've deleted the Mapping Model and the new version of the model, cleaned, then recreated both.
• I've tried making a different change in the new model - deleting an entity instead.

I'm at my wits end.

I can't help but think I've made a huge mistake somewhere that I'm not seeing. Any ideas?

Answer 1

Two possibilities:

1. Your source model in your app does not match the actual store on disk.
2. Your mapping model does not match your source model.

Turn on Core Data debugging and you should be able to see the hashes that Core Data is looking for when it is doing the migration. Compare these hashes to what is in your store on disk and see if they match up. Likewise the debugging should let you see the hashes in the mapping model to help you match everything up.

If it is just your mapping model that is misaligned, you can tell it to update from source from the design menu in Xcode. If you are missing the actual source model for your store file on disk then you can look in your version control system or try using an automatic migration to get that file to migrate to the model that you believe is the source.

Answer 2

Rather than merging all models in the bundle, I've specified the two models I want to use (model 1 and the new version of model 2) and merged them using modelByMergingModels:

This doesn't seem right. Why merge the models? You want to use model 2, migrating your store from model 1.

From the NSManagedObjectModel class reference

modelByMergingModels:

Creates a single model from an array of existing models.

You don't need to do anything special/specific with your source model (model 1).. just so long as it's in your bundle, the automatic lightweight migration process will discover and use it.

I would suggest abandoning the mapping model you created in XCode, as I've seen terrible performance in comparison with the automatic-lightweight migrations. Your mileage may vary, my changes between models are different to yours, but i wouldn't be surprised. Try some timing with and without your own mapping model in the bundle.

 /* Inferred mapping */
 NSError *error;
 NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                          [NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
                          [NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption,nil];
 NSPersistentStore *migratedStore = [persistentStoreCoordinator addPersistentStoreWithType:nil
                                                                             configuration:nil
                                                                                       URL:self.storeURL
                                                                                   options:options
                                                                                     error:&error];
 migrationWasSuccessful = (migratedStore != nil);

You can verify in your code that your source model is available, by attempting to load it and verify that it is not nil:

NSString *modelDirectoryPath = [[NSBundle mainBundle] pathForResource:@"YourModelName" ofType:@"momd"];
if (modelDirectoryPath == nil) return nil;
NSString *modelPath = [modelDirectoryPath stringByAppendingPathComponent:@"YourModelName"];
NSURL *modelFileURL = [NSURL fileURLWithPath:modelPath];
NSManagedObjectModel *modelOne = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelFileURL];
if (modelOne == nil) {
    NSLog(@"Woops, XCode lost my source model");
}
else {
    [modelOne release];
}

This assumes in your project you have a resource "YourModelName.xcdatamodeld" and "YourModelName.xcdatamodel" within it.

---

Also, you can check if that model is compatible with your existing, pre-migration persistent store:

NSError *error;
NSDictionary *storeMeta = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:nil URL:self.storeURL error:&error];
if (storeMeta == nil) {
    // Unable to read store meta
    return NO;
}
BOOL isCompatible = [modelOne isConfiguration:nil compatibleWithStoreMetadata:storeMeta];

That code assumes you have a method -storeURL to specify where the persistent store is loaded from.