Objective-C Fundamentals

252 CHAPTER 12 Reading and writing application data
optional), Core Data does most of the work for you. Such migrations are called lightweight
migrations. Let’s see what this looks like in practice: you’ll add a new property—
age—to the Person entity. To do that, you must first create a new version of your data
model. Select the PocketTasks.xcodedatamodel, then select Editor > Add Model Version
from the menu. A new file called PocketTasks 2.xcdatamodel will appear in the
PocketTasks.xcdatamodeld directory. This new version should be used by your application
from now on. For that to take effect, select PocketTasks.xcdatamodeld and
choose “PocketTasks 2” from the “Current” select box in the “Versioned Data Model”
section of the first tab of the Utilities pane. The file will get a little green checkmark.
Now add the new age property to the Person entity in the new version you just created.
Make it optional, and set the type to Integer16. Save it and build and run. The
application will crash and report errors in the Console: the saved data is incompatible
with the new data model! You have to tell Core Data to automatically migrate it for
you. Open PocketTasksAppDelegate.m and find the persistentStoreCoordinator
method. Change it to look like the following listing.
Listing 12.17
Automatically migrate a data model in PocketTasksAppDelegate.m
- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
if (__ persistentStoreCoordinator != nil) {
return __persistentStoreCoordinator;
}
NSURL *storeURL =
[[self applicationDocumentsDirectory]
URLByAppendingPathComponent:@"PocketTasks.sqlite"];
NSDictionary *options =
[NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES],
NSMigratePersistentStoresAutomaticallyOption,
[NSNumber numberWithBool:YES],
NSInferMappingModelAutomaticallyOption, nil];
NSError *error = nil;
__persistentStoreCoordinator =
[[NSPersistentStoreCoordinator alloc]
initWithManagedObjectModel:[self managedObjectModel]];
if (![__persistentStoreCoordinator
addPersistentStoreWithType:NSSQLiteStoreType
configuration:nil
URL:storeURL
options:options
error:&error])
{
NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
abort();
}
}
return __persistentStoreCoordinator;