How do I set the value of an inverse relationship in a mapping model?

Question

In v1 of my model I have a one to many relationship between a presentation and slides:

presentation---->>slides

In v2 I fix this foolishness by adding an inverse one to one:

presentation<--->>slides

In both models a slide belongs to 1 and only 1 presentation (although this is not codified within the models).

How do I specify the value expression in the mapping model to create the slide-->presentation relationship without creating an NSEntityMigrationPolicy subclass?

Answer 1

As far as I can tell it is not possible to do this with a Value Expression in the mapping. The solution is to create a NSEntityMigrationPolicy subclass for the mapping.

In the NSEntityMigrationPolicy subclass override createRelationshipsForDestinationInstance:entityMapping:manager:error:. In this method fetch the related object with a standard NSFetchRequest executed in the managers destinationContext. Be sure to call super to ensure that any other relationships are also migrated.