core Data deletes data after restarting the application

Question

I changed my structure and made a migration (I created a new Model), the data is added and displayed, everything works fine, but when I reload the application, the core data is empty, what's the problem?

  static func saveToCoreData(_ words: Words?){

    DispatchQueue.main.async {

        coreDataResult(data: [words?.word, words?.translation], completion: { (result, taskObject, context) in
            do {
                let fetchedEntities = try context.fetch(result) as! [Favorite]
                if let _ = fetchedEntities.first?.word {
                    print("the element already have in coreData")
                } else {

                    taskObject?.setValue(words?.word, forKey: "word")
                    taskObject?.setValue(words?.translation, forKey: "translation")
                    taskObject?.setValue(words?.descript, forKey: "wordDesc")
                    taskObject?.setValue(words?.translDesc, forKey: "translDesc")

                    do {
                        try context.save()
                    } catch {
                        print(error)
                    }
                }
            } catch {}
        })

    }

}

static func coreDataResult(data: [[String?]?]?, completion: @escaping (NSFetchRequest<NSFetchRequestResult>, Favorite?, NSManagedObjectContext) -> ()){

    guard let w = data?.first, let word = w, let t = data?.last, let transl = t else { return }

    DispatchQueue.main.async {
        let appDelegate = UIApplication.shared.delegate as! AppDelegate
        let context = appDelegate.persistentContainer.viewContext
        guard let entity = NSEntityDescription.entity(forEntityName: "Favorite", in: context) else { return }
        guard let taskObject = NSManagedObject(entity: entity, insertInto: context) as? Favorite else { return }

        let predicate = NSPredicate(format: "word == %@", word)
        let predicate2 = NSPredicate(format: "translation == %@", transl)
        let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Favorite")
        let andPredicate = NSCompoundPredicate(type: .and, subpredicates: [predicate, predicate2])

        fetchRequest.predicate = andPredicate
        completion(fetchRequest, taskObject, context)
    }

}

old attributes

new Attributes

new Model

You changed the data type of the attribute "word" from String to Transformable. Lightweight migration cannot do this automatically. You need to do that manually. — MartinM, Mar 13 '19 at 14:49
You would need to write a migration policy by the looks of it — Sean Lintern, Mar 13 '19 at 14:55

Vasya2014 · Accepted Answer · 2019-03-15T08:20:30.650

I decided it

this answer helped me - https://stackoverflow.com/a/40662940/8070211

create Model (command + n -> Mapping Model -> select old model -> select new model -> add Name and create)

create subClass NSEntityMigrationPolicy

class FavoriteEntityMigrationPolicy: NSEntityMigrationPolicy {

   @objc func typeFor(word:String) -> [String] {
       return [word]
   }

}

select Model -> select entity -> add subClass to custom Policy
select Model -> select entity -> select value expression -> add FUNCTION($entityPolicy, "typeForWord:" , $source.word)
it is ready

core Data deletes data after restarting the application

1 Answers1