How to batch delete in Core Data

Issue #622

If the entities that are being deleted are not loaded into memory, there is no need to update your application after the NSBatchDeleteRequest has been executed. However, if you are deleting objects in the persistence layer and those entities are also in memory, it is important that you notify the application that the objects in memory are stale and need to be refreshed.

To do this, first make sure the resultType of the NSBatchDeleteRequest is set to NSBatchDeleteRequestResultType.resultTypeObjectIDs before the request is executed. When the request has completed successfully, the resulting NSPersistentStoreResult instance that is returned will have an array of NSManagedObjectID instances referenced in the result property. That array of NSManagedObjectID instances can then be used to update one or more NSManagedObjectContext instances.

let fetchRequest: NSFetchRequest<NSFetchRequestResult> = Book.fetchRequest()
let deleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
deleteRequest.resultType = .resultTypeObjectIDs

do {
    let context = CoreDataManager.shared.container.viewContext
    let result = try context.execute(
        deleteRequest
    )

    guard
        let deleteResult = result as? NSBatchDeleteResult,
        let ids = deleteResult.result as? [NSManagedObjectID]
    else { return }

    let changes = [NSDeletedObjectsKey: ids]
    NSManagedObjectContext.mergeChanges(
        fromRemoteContextSave: changes,
        into: [context]
    )
} catch {
    print(error as Any)
}