I have a scenario where I'm moving the contents of a blob stored in a core data entity into a file. I need a way to export that data during a migration, where I know the entity that's being converted and save the blob to a file, writing the location of that file into the converted entity's appropriate attribute.

I can't seem to find a way to do this. The docs regarding the Three Stage Migration seem to indicate what can be done, but I'm not sure where to define things, or what exactly to define.