Using mergedModelFromBundles: and versioning (CoreData)

Question

I'm trying to use the migration feature in CoreData. I've followed the Apple Documentation. I have a problem in the following method:

/**
 Returns the managed object model for the application.
 If the model doesn't already exist, it is created by merging all of the models found in the application bundle.
 */
- (NSManagedObjectModel *)managedObjectModel {

    if (managedObjectModel != nil) {
        return managedObjectModel;
    }
    /* 
     * NSInvalidArgumentException', reason: '*** -[NSCFArray insertObject:atIndex:]: attempt to insert nil'
     * 2010-02-17 16:27:15.338 Patrimoine[3037:207]
     */ 
    managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles:nil] retain];    
    return managedObjectModel;
}

It appears that there is the same problem on http://iphonedevelopment.blogspot.com/2009/09/core-data-migration-problems.html

Yet I did choose the method Apple suggests, by using the menu option "Add Model Version".

Do you have any idea?

Answer 1

You need to clean your project. As soon as you "version" your model Xcode moves it into a bundle (folder) but it does not remove the old one. What happens then is that the next time you run your app there are two copies of your model in the bundle; the old one and the new one that is inside of the momd bundle.

Doing a Project -> Clean All will resolve this issue.

Answer 2

Also, if you rename your models at any point, be sure and repeat the "set current model" step by switching to an older then back to the newer model again. My build setting would not reset itself automatically and kept setting the "current model name" to a nonexistent model, resulting in the exact same problem.

You can always verify this setting is correct in the build product's resource folder, inside the imported .momd directory, in a file called versioninfo.plist - the setting for the current model MUST match the actual name of your model.

Answer 3

I've found that using the mergedModelFromBundel: method doesn't seem to work with migration; I switched to -initWithContentsOfURL:, and it works fine. Note that you have to init it with a URL pointing to a ".momd" file.

Answer 4

If you change your data model at any point, be sure to Reset Content and Settings in the simulator, or you will get the error that the version of the model used to make the store is not the same as the model used to access it.