Versioning and Migration - Core Data - page 40

Figure 8—Setting the current model version
Now that we have a new version of the MOM, it's time to add the new entities
and attributes. This process requires the addition of a new entity and some
changes to the Recipe entity. Comparing the v1 data model and the v2 data
model ( Figure 9, Versions 1 and 2 of the data model , on page 43 ), you can see
that we added a new Author entity along with its one-to-many relationship with
the Recipe entity. Also, the Recipe entity has a new attribute called lastUsed , which
is defined as a Date .
We're not quite done. If we were to run the application right now, we would
trip the error that we discussed in Section 3.1, Some Maintenance Before We
Migrate , on page 38 . Clearly, something is missing.
Turning on Automatic Data Migration
The first thing we need to do is to tell Core Data to automatically migrate data
when the persistent store is not using the same model as the current version.
To do this, we need to make a small change to the persistentStoreCoordinator
method in our AppDelegate . Previously we were passing nil to the addPersistentStore-
WithType:configuration:URL:options:error: method for the options parameter. However,
we need to change that to the following: