NSStagedMigrationManager shortcomings

It seems to me that NSStagedMigrationManager has algorithmic issues. It doesn't perform staged migration, if all its stages are NSLightweightMigrationStage.

You can try it yourself. There is a test project with three model versions V1, V2, V3, V4. Migrating V1->V2 is compatible with lightweight migration, V2->V3, V3->V4 is also compatible, but V1->V3 is not. I have following output:

Migrating V1->V2, error: nil

Migrating V2->V3, error: nil

Migrating V3->V4, error: nil

Migrating V1->V3, no manager, error: Optional("Persistent store migration failed, missing mapping model.")

Migrating V1->V3, lightweight[1, 2, 3], error: Optional("Persistent store migration failed, missing mapping model.")

Migrating V1->V3, lightweight[1]->lightweight[2]->lightweight[3], error: Optional("Persistent store migration failed, missing mapping model.")

Migrating V1->V3, custom[1->2]->lightweight[3], error: nil

Migrating V1->V3, lightweight[1]->custom[2->3], error: nil

Migrating V1->V3, custom[1->2]->custom[2->3], error: nil

Migrating V1->V4, error: Optional("Persistent store migration failed, missing mapping model.")

Migrating V2->V4, error: nil

Migrating V1->V4, custom[1->2]->lightweight[3, 4], error: nil

Migrating V1->V4, lightweight[3, 4]->custom[1->2], error: Optional("A store file cannot be migrated backwards with staged migration.")

Migrating V1->V4, lightweight[1, 2]->lightweight[3, 4], error: Optional("Persistent store migration failed, missing mapping model.")

Migrating V1->V4, lightweight[1]->custom[2->3]->lightweight[4], error: nil

Migrating V1->V4, lightweight[1,4]->custom[2->3], error: nil

Migrating V1->V4, custom[2->3]->lightweight[1,4], error: Optional("Persistent store migration failed, missing mapping model.")

I think that staged migration should satisfy the following rules for two consecutive stages:

1. Any version of lightweight stage to any version of lightweight stage;
2. Any version of lightweight stage to current version of custom stage;
3. Next version of custom stage to any version of lightweight stage;
4. Next version of custom stage to current version of custom stage.

However, rule 1 doesn't work, because migration manager skips intermediate versions if they are inside lightweight stages, even different ones.

Note that lightweight[3, 4]->custom[1->2] doesn't work, lightweight[1,4]->custom[2->3] works, but custom[2->3]->lightweight[1,4] doesn't work again.

Would like to hear your opinion on that, especially, from Core Data team, if possible.

Thanks!