I'm using Core Data's automatic lightweight migration successfully. However, when a particular entity gets created during a migration, I'd like to populate it with some data. Of course I could check if the entity is empty every time the application starts, but this seems inefficient when Core Data has a migration framework.
Is it possible to detect when a lightweight migration occurs (possibly using KVO or notifications), or does this require implementing standard migrations?
I've tried using the NSPersistentStoreCoordinatorStoresDidChangeNotification
, but it doesn't fire when migrations occur.
To detect whether a migration is needed, check to see if the persistent store coordinator's managed object model is compatible with the existing store's metadata (adapted from Apple's Is Migration Necessary):
NSError *error = nil;
persistentStoreCoordinator = /* Persistent store coordinator */ ;
NSURL *storeUrl = /* URL for the source store */ ;
// Determine if a migration is needed
NSDictionary *sourceMetadata = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:NSSQLiteStoreType
URL:storeUrl
error:&error];
NSManagedObjectModel *destinationModel = [persistentStoreCoordinator managedObjectModel];
BOOL pscCompatibile = [destinationModel isConfiguration:nil compatibleWithStoreMetadata:sourceMetadata];
NSLog(@"Migration needed? %d", !pscCompatibile);
If pscCompatibile
is NO
, then a migration will need to occur. To examine the entity changes, compare the NSStoreModelVersionHashes
key in the sourceMetadata
dictionary to the [destinationModel entities]
:
NSSet *sourceEntities = [NSSet setWithArray:[(NSDictionary *)[sourceMetadata objectForKey:@"NSStoreModelVersionHashes"] allKeys]];
NSSet *destinationEntities = [NSSet setWithArray:[(NSDictionary *)[destinationModel entitiesByName] allKeys]];
// Entities that were added
NSMutableSet *addedEntities = [NSMutableSet setWithSet:destinationEntities];
[addedEntities minusSet:sourceEntities];
// Entities that were removed
NSMutableSet *removedEntities = [NSMutableSet setWithSet:sourceEntities];
[removedEntities minusSet:destinationEntities];
NSLog(@"Added entities: %@\nRemoved entities: %@", addedEntities, removedEntities);