Lightweight Migration
Whenever a new model is set as the current version, existing persistent stores must be migrated to use them. This is because the persistent store coordinator will try to use the latest model to open the existing store, which will fail if the store was created using a previous version of the model. The process of store migration can be handled automatically by passing the following options to a persistent store coordinator in an NSDictionary as a store is added to it:
When NSMigratePersistentStoresAutomaticallyOption is YES and passed to a persistent store coordinator, Core Data will automatically attempt to migrate lower ...
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