Core data Migration Delete data of entity

Question

I want to do Lightweight migration in core-data. I am adding an attribute to an entity which is working well.

But I want after this particular migration that entity's data (all objects contained in that entity's table) to be deleted.

I gone through this question, but this method is not looking good as I wanted to keep separate the logic of each migration which will be required in future.

I gave a thought to one way that directly rename that entity but not specifying the rename Identifier so that Core data will process it as deletion of an entity and addition of a new entity, but this thing will not become permanent solution for every similar case in future migrations.

Is there any way if I can delete data directly by going through UI of xcdatamodeld or is there any other method?

Answer 1

I was able to do this, after much frustration of finding a way, using a custom EntityMigrationPolicy via a Mapping Model, setting the Custom Policy for the entity mapping, eg. EntityNameToEntityName, to this policy (ProductName.DeleteEntityPolicy):

// Swift 5
class DeleteEntityPolicy: NSEntityMigrationPolicy {
    override func begin(_ mapping: NSEntityMapping, with manager: NSMigrationManager) throws {
        // Get all current entities and delete them before mapping begins
        let entityName = "EntityName"
        let request = NSFetchRequest<NSManagedObject>(entityName: entityName)
        let context = manager.sourceContext
        let results = try context.fetch(request)
        results.forEach(context.delete)
        try super.begin(mapping, with: manager)
    }
}

More info of approach to set up a custom migration with a mapping model: https://stackoverflow.com/a/40662940

Would love to know if there's a better way/built in way to do this.