I have a cocoa application using core-data, which is now at the 4th version of its managed object model.

My managed object model contains abstract entities but so far I have managed to get migration working by creating appropriate mapping models and creating my persistent store using addPersistentStoreWithType:configuration:options:error and with the NSMigratePersistentStoresAutomaticallyOption set to YES.

NSDictionary *optionsDictionary = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:NSMigratePersistentStoresAutomaticallyOption];
NSURL *url = [NSURL fileURLWithPath: [applicationSupportFolder stringByAppendingPathComponent: @"MyApp.xml"]];
NSError *error=nil;
[theCoordinator addPersistentStoreWithType:NSXMLStoreType configuration:nil URL:url options:optionsDictionary error:&error]

This works fine when I migrate from model version 3 to 4, which is a migration that involves adding attributes to several entities. Now when I try to add a new model version (version 5), the call to addPersistentStoreWithType returns nil and the error remains empty. The migration from 4 to 5 involves adding a single attribute.

I am struggling to debug the problem and have checked all the following;

1. The source database is in fact at version 4 and the persistentStoreCoordinator's managed object model is at version 5.

2. The 4->5 mapping model as well as managed object models for versions 4 and 5 are present in the resources folder of my built application.

3. I've tried various model upgrade paths. Strangely I find that upgrading from an early version 3 -> 5 works .. but upgrading from 4 -> 5 fails.

4. I've tried adding a custom entity migration policy for migration of the entity whose attributes are changing ... in this case I overrode the method beginEntityMapping:manager:error: . Interestingly this method does get called when migration works (ie when I migrate from 3 to 4, or from 3 to 5 ), but it does not get called in the case that fails ( 4 to 5 ).

I'm pretty much at a loss as to where to proceed. Any ideas to help debug this problem would be much appreciated.