Core Data migration "can't merge models with two different entities named 'foo'" - iphone

Question

I know this is a frequently asked question, however none of the solutions that I have found seem to work for me.
This is my situation:
I have one data model for my application, and I wanted to add versioning to it. So in XCode, I did Design -> Data Model -> Add Model Version. I also updated my delegate's persistentStoreCoordinator method to look like this:
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
if (persistentStoreCoordinator != nil) {
return persistentStoreCoordinator;
}
NSURL *storeUrl = [NSURL fileURLWithPath: [[self applicationDocumentsDirectory]
stringByAppendingPathComponent: #"foo.sqlite"]];
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES], NSMigratePersistentStoresAutomaticallyOption,
[NSNumber numberWithBool:YES], NSInferMappingModelAutomaticallyOption,
nil];
NSError *error = nil;
persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
if(![persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeUrl options:options error:&error]) {
/*Error for store creation should be handled in here*/
}
return persistentStoreCoordinator;
}
Just to make sure everything was still working, I did a clean all, build, and tested it in the simulator. Everything worked so far.
Next I chose the new version data model, set it to be the current version using XCode, and added one extra attribute to an entity. I then did a clean all, build. And now whenever I start the application it crashes with this error: 'Can't merge models with two different entities named 'foo''.
What am I doing wrong? I have tried making sure that no data model is added to the target, adding just the current version data model to the target, and both. Every time I test I make sure to clean all.
Can anyone shed some light as to why it does not work for me?
EDIT:
here is my managedObjectModel method:
- (NSManagedObjectModel *)managedObjectModel {
if (managedObjectModel != nil) {
return managedObjectModel;
}
managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles:nil] retain];
return managedObjectModel;
}

Answer 1

I already expected this implementation of the managedObjectModel getter.
In your implementation all models within the bundle are merged into one single model. Therefore also all versions within the .momd are merged resulting in duplicate entity definitions.
Change the code to explicitly initialize the model with the apropriate model file and it should work fine.
NSString *modelPath = [[NSBundle mainBundle] pathForResource:#"datamodel" ofType:#"momd"];
NSURL *modelURL = [NSURL fileURLWithPath:modelPath];
managedObjectModel_ = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];

Answer 2

If you're using Version Models on Core Data you always have to initialize to the version of the model that you want to use. On the Application Bundle you will find one file with extension .momd that is the Full Model. Inside of this file you will find a lot of .mom files inside, each .mom file represent one version of your model.
If you run your application and initialize with the .momd file and all versions inside, Core Data will create all versions and later we will have the "duplicated entity" error, Core Data don't known what version use. Now, the only way to fix the problem is delete the app, point your code to the correct .mom file and run again, so Core Data create only one version of the internal database.
Here's one snippet of code that do the job:
NSString *fullModelName = #"myModel.momd"; // The name of the main model.
NSString *modelVersionName = #"myModel1.0.mom"; // Only the name of the version.
NSString *bundlePath = [[NSBundle mainBundle] bundlePath];
NSString *modelPath = [NSString stringWithFormat:#"%#/%#/%#", bundlePath, fullModelName, modelVersionName];
//
NSURL *modelURL = [NSURL fileURLWithPath:modelPath];
managedObjectModel_ = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];