Migrating complex core data model

3
votes

I have a couple of questions about core data model migration.

I have a pretty complex data model with a couple cases of entity inheritance. I was going to make some changes to the data model in a new version and try and setup migration but when it migrated the store I lost some of the data that belonged to an entity that inherited from another entity.

In my case I have a few entities that all inherit from a "Resource" entity. This resource entity has a attribute "name". When I try to migrate the data store all entities that inherit from the "Resource" entity lose their name.

Is their any way to get model migration working for a data model with inheritance? I have already shipped a beta and I need to make a couple of updates to the model but I obviously don't want the users to lose all of their data.

Thanks

1
cocoacore-data

1 Answers

0
votes

Try "playing" on your new model with Column properties > Versioning > Renaming identier, entering the previous field name, which I guess is the same. I doubt that will work with inheritance, but that is worth the try... (That not so documented feature, allowing to keep data across renames, saved me several times).

If that doesn't work, I'm afraid you have to do a "manual migration"... with Model mappings and other things... which is imho a bit complex. See Apple documentation on this topic... I then would suggest to just rollback your changes and forget inheritance, quicker & easier, even if it is less "clean". Or just assume your users will loose some data, at beta stage this is not so important... (Or maybe you can just collect old data in memory/plist file before migrating model an then repopulate)

Good luck! CoreData automatic model migration is great, but take care that it will work only with simple modifications...

Oh, just one another trick, add -com.apple.CoreData.SQLDebug 1 to your app launch arguments, and you will get all sql requests generated by CoreData... That might help you to understand the migration process. (and some other things...)