Core Data Migration Across Multiple Version Upgrades

Question

I have an iPhone app that uses Core Data.

I did an update and used Lightweight Migration to go from V1 to V2 of my MOM (Managed Object Model). This worked perfectly.

What happens when I want to go to V3 (and beyond) of my MOM?

• If I decide to continue with Lightweight Migration, will it automatically deal with migrating from V1 to V3 and V2 to V3 of my MOM, or do I need to do something extra?
• If I decide to use a mapping model, what happens? How do I deal with upgrading both V1 and V2 MOM's to V3? Do I need to create a mapping model for both V1 to V3 and V2 to V3?
• This question goes further ... what happens when I have V6 MOM and still need to support the possibility of upgrading from a V1 MOM?

Another question is what is the best way to determine the version of the current MOM? Should I use isConfiguration:compatibleWithStoreMetadata:

Thanks for any assistance. I am loving Core Data. But it sometimes makes my head spin and I get confused, which is why I am seeking some sage wisdom.

Answer 1

I went with ordinary migration using createDestinationInstancesForSourceInstance.
The snippet shows how to override that method and how to get the sourceVersion of the model to migrate. The actual migration is happening in the helper class TZMigrationHelper.

- (BOOL)createDestinationInstancesForSourceInstance:(NSManagedObject *)sInstance entityMapping:(NSEntityMapping *)mapping manager:(NSMigrationManager *)manager error:(NSError **)error
{
    float sourceVersion = [[[mapping userInfo] valueForKey:@"sourceVersion"] floatValue];
    if(sourceVersion <= 0.9)
    {
     mapping = [TZMigrationHelper addAttributeMappingForDerivedRTFProperties:sInstance mapping:mapping propertyName:@"someProperty"];
     mapping = [TZMigrationHelper addAttributeMappingForDerivedRTFProperties:sInstance mapping:mapping propertyName:@"anotherProperty"];
     mapping = [TZMigrationHelper addAttributeMappingForDerivedRTFProperties:sInstance mapping:mapping propertyName:@"oneMoreProperty"];  
    }
    return [super createDestinationInstancesForSourceInstance:sInstance entityMapping:mapping manager:manager error:error];
}

Answer 2

The initial posting was now many months ago, but I think that the best answer is found in Marcus Zarra's Core Data book (or online in the code examples). Google for "progressivelyMigrateURL" and one will find code for progressively iterating through models - which would allow one to create mappings from model n to model n+1, while not worrying about the combinatorial explosion for creating mappings between all pairings of models.

This may result in a slower migration at run time. I haven't investigated this.