Detecting a Lightweight Core Data Migration

Question

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.

Answer 1

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);

Answer 2

The accepted answer converted to Swift...

  var persistentStoreCoordinator: NSPersistentStoreCoordinator?
  var url: URL
  do {
    let sourceMetadata = try NSPersistentStoreCoordinator.metadataForPersistentStore(ofType: NSSQLiteStoreType, at: url, options: nil)
    if let destinationModel = persistentStoreCoordinator?.managedObjectModel {
      let compatibile = destinationModel.isConfiguration(withName: nil, compatibleWithStoreMetadata: sourceMetadata)
      if !compatibile {
        if let versionHashes = sourceMetadata["NSStoreModelVersionHashes"] as? [String: Any] {
          let sourceEntities = Set(versionHashes.keys)
          let destinationEntities = Set(destinationModel.entitiesByName.keys)

          var addedEntities = Set(destinationEntities)
          addedEntities.subtract(sourceEntities)

          var removedEntities = Set(sourceEntities)
          removedEntities.subtract(destinationEntities)
          let modelName = (destinationModel.versionIdentifiers.first as? String) ?? ""
          NSLog("Core Data requires a migration to model '\(modelName)'...\nAdded: \(addedEntities)\nRemoved: \(removedEntities)")
        }
      }
    }
  } catch {
            ...
  }

Answer 3

What about subclassing NSManagedObject for that entity, and then overriding -awakeFromInsert:? Or are you creating this entity in other parts of your app?