Core Data migration fails for to-one relationship

问题:

I have the following model:

parent is a to-one relationship (optional, no minimum, maximum=1). children is the inverse to-many relationship (optional, no minimum, no maximum).

I added a new model version, where only the Entity Room is renamed to Classroom, all other entities are unchanged:

I also created a mapping model for the migration, but the migration fails with the error message

reason = "Can't find mapping model for migration";

The strange thing is that if the parent relationship is created with minimum=1 (instead of no minimum) then the migration works without problems.

The persistent store is opened with the options

NSDictionary *options = @{NSMigratePersistentStoresAutomaticallyOption:@YES, NSInferMappingModelAutomaticallyOption:@NO};

回答1:

Why do you need a mapping model? You should avoid these complexities and simply use Lightweight Migration as explained in Apple's Lightweight Migration Documentation.