As Property Manager acquires new features (in this case, additional tagging and notation facilities), the data model needs to be updated. This in turn depends on CoreData to migrate the user’s previous documents to the latest data model. Traditionally, CoreData migrates the data transparently when the user opens the file, however here lies the problem with sandboxing: the updated file cannot be saved to disk unless the user is presented a
NSSavePanel. Because of this, automatic migrations will fail.