Obviously there is no magic involved in the data migration process, and although there are no major code changes required to use Core Data migration, it does require a mapping document that explains to the framework how to translate the data from one model to the other. That document is called the mapping model.
A mapping model can range from the very simple with minor changes to the underlying data structure to extremely complex changes that require custom code. This article focuses on the less-complex translations of data. To create a mapping model, you start off by creating the new version of your data model.
To create a mapping model, select File -> New File from the main menu and select Design [Mapping Model] from the list. Once you name the mapping model, you then select the source model and the destination model that will be mapped.
At this point, XCode will do its best to guess how the mapping should be. At this point, you will need to verify that the document is correct and adjust it accordingly.