Anyone who has never made a mistake has never tried anything new.
In Chapter 2, “Managed Object Model Basics,” the fundamentals of managed object models were introduced, yet you were constrained to just one entity and a few attributes. The next logical step is to add to the model; however, this requires a number of preliminary steps to prevent errors caused by these changes. This chapter shows how to add model versions and model mappings, and it demonstrates different migration techniques you can choose when upgrading a model.
As an application evolves, its managed object model probably needs to change, too. Simple changes, such as attribute defaults, validation ...