In my recent endeavor, I aimed to introduce new Fetch Index Elements to the Core Data model of my iOS application. To achieve this, I followed a process of lightweight migration, detailed as follows:
- Navigate to Editor > Add Model Version to create a new version of the data model.
- Name the new version with a sequential identifier (e.g., MyAppModelV3.xcdatamodel) based on the naming convention of previous models.
- Select the newly created version, MyAppModelV3.xcdatamodel, as the active model.
- Mark this new version as the "Current" model in the Xcode properties panel on the right.
- In the new version of the model, MyAppModelV3.xcdatamodel, and add the new Fetch Index Elements there. Also, insert "v3" in the Versioning Hash Modifier field of affected entity, to indicate this modification.
Upon reflection, I realized that creating a new version of the xcdatamodel might not have been necessary for this particular case. However, it appears to have caused no adverse effects on the application's functionality.
During testing, I executed the application in a simulated environment, initially running an older version of the app to inspect the database content with SQLite DB Browser. I then upgraded to the latest app version to verify that the migration was successfully completed without causing any crashes. Throughout this testing phase, I employed the -com.apple.CoreData.MigrationDebug 1 flag to monitor all SQL operations, ensuring that indexes were appropriately dropped and recreated for the affected entity.
Following thorough testing, I deployed the update to production. The majority of users were able to upgrade to the new app version seamlessly. However, a small fraction reported crashes at startup, indicated by the following error message:
Fatal error: Unresolved error Error Domain=NSCocoaErrorDomain Code=134110 "An error occurred during persistent store migration." UserInfo={NSUnderlyingError=0x2820ad3e0 {Error Domain=NSCocoaErrorDomain Code=134100 "The managed object model version used to open the persistent store is incompatible with the one that was used to create the persistent store." UserInfo={metadata={ NSPersistenceFrameworkVersion = 1338; NSStoreModelVersionChecksumKey = "qcPf6+DfpsPrDQ3j1EVXcBIrFe1O0R6IKd30sJf4IrI="; NSStoreModelVersionHashes = { NSAttachment = {length = 32, ...
Strangely, the only way I could replicate this issue in the simulator was by running the latest version of the app followed by reverting to an older version, a scenario unlikely to occur in a real-world setting. This raises the question: How could this situation arise with actual users, considering they would typically move from an old to a new version rather than the reverse?
I am reaching out to the community for insights or advice on this matter. Has anyone else encountered a similar problem during the Core Data migration process? How did you resolve it?
