核心数据强制自定义映射模型/策略而不是轻量级迁移

I've got about 4 different versions of my data model now, and every one except the last one has been just a minor change using automatic lightweight migration. For this latest model, I need to do a bit of additional work during the migration, so I created a custom mapping model and a migration policy subclass with some actions in the createDestinations and createRelationships. Problem is, my mapping model/custom policy isn't being called, and it seems that Core Data is trying to perform lightweight migration instead. Is there something I need to do to keep lightweight migration around, but use my mapping model when there's one available?I've got about 4 different versions of my data