Recently for a major update of an app I’m working on, I had to use a Core Data Migration Manager for the first time.

I run into a weird problem with some pragma statements set in the underlying SQLite database, so I’d like to offer a workaround in case someone else runs into this issue in the future.

The setup

Here’s the setup of the Core Data stack in our app:

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }
    
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"CDMigration.sqlite"];
    
    NSError *error = nil;
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    
    NSDictionary *options = @{NSMigratePersistentStoresAutomaticallyOption:@YES,
                              NSInferMappingModelAutomaticallyOption:@YES,
                              NSSQLitePragmasOption: @{@"journal_mode": @"WAL"}
                              };
    
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:options error:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }    
    
    return _persistentStoreCoordinator;
}

It’s nothing fancy: the stock Core Data initialization you get with the Xcode template, plus the two options that are commented out (NSMigratePersistentStoresAutomaticallyOption and NSInferMappingModelAutomaticallyOption) to enable migrations, and a setting to put SQLite in journal_mode = WAL.

I’m not really sure I need this journal_mode set, but since our app started using MagicalRecord, that’s the way it was setup. Here’s the relevant SQLite documentation on journal modes, so you can make an informed decision.

The problem

When you use a Migration Manager, Core Data will create a new database for you, and start copying the entities one by one from the old DB to the new one.

As we are using journal_mode = WAL, there’s an additional file besides DB.sqlite called DB.sqlite-wal.

From what I can tell, the problem seems to be that Core Data creates a temporary DB, inserts everything there, and when it renames it to the original name, the -wal file is kept as a leftover from the old version. The problem is that you end up with an inconsistent DB.

Workaround

As I’m not sure I want WAL enabled, I decided to keep the setting and implement a workaround. It’s simple:

  1. Detect if you need a migration.
  2. If you do, set the journal_mode to DELETE.
  3. Initialize the NSPersistentStoreCoordinator, so it triggers the migrations.
  4. After a successful initialization, recreate the NSPersistentStoreCoordinator with journal_mode set to WAL, as it was.

Here’s how the code looks:

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator
{
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }
    
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"CDMigration.sqlite"];
    
    NSError *error = nil;
    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    
    NSDictionary *options = @{NSMigratePersistentStoresAutomaticallyOption:@YES,
                              NSInferMappingModelAutomaticallyOption:@YES,
                              NSSQLitePragmasOption: @{@"journal_mode": @"WAL"}
                              };
    
    // Check if we need a migration
    NSDictionary *sourceMetadata = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:NSSQLiteStoreType URL:storeURL error:&error];
    NSManagedObjectModel *destinationModel = [_persistentStoreCoordinator managedObjectModel];
    BOOL isModelCompatible = (sourceMetadata == nil) || [destinationModel isConfiguration:nil compatibleWithStoreMetadata:sourceMetadata];
    if (! isModelCompatible) {
        // We need a migration, so we set the journal_mode to DELETE
        options = @{NSMigratePersistentStoresAutomaticallyOption:@YES,
                    NSInferMappingModelAutomaticallyOption:@YES,
                    NSSQLitePragmasOption: @{@"journal_mode": @"DELETE"}
                    };
    }

    NSPersistentStore *persistentStore = [_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:options error:&error];
    if (! persistentStore) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }    
    
    // Reinstate the WAL journal_mode
    if (! isModelCompatible) {
        [_persistentStoreCoordinator removePersistentStore:persistentStore error:NULL];
        options = @{NSMigratePersistentStoresAutomaticallyOption:@YES,
                    NSInferMappingModelAutomaticallyOption:@YES,
                    NSSQLitePragmasOption: @{@"journal_mode": @"WAL"}
                    };
        [_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:options error:&error];
    }

    
    return _persistentStoreCoordinator;
}

Radar or GTFO1

I’ve filed a bug report to Apple on this issue. If you run into this as well, feel free to duplicate it.

  1. http://blackpixel.com/blog/2012/02/radar-or-gtfo.html ↩