some facts...
- core data store can be accessed only by object model created it.
- change in object model makes it incompatible with store created by it previously.
- so some way needed to change store format according to change in model, where migration comes in to scene.
- for migration, each model assigned its version. so versioned model consists of more then one models with different versions.
- we need version of current model and new model where we want to migrate our store.
- core data uses current version of model and can open persistant stores created by any models and can migrate store according to current version.
- though some info needed to how to map one mode to another in form of hints in versioned model itself or separate mapping file.
