Enabling core data lightweight migration in Swift 3

Question

According to the articles I have read the correct way to enable core data light weight migrations is by passing options to addPersistentStoreWithType:

Answer 1

You do it using NSPersistentStoreDescription, which is where all those options moved to in the Swift 3 updates. Do this before the call to loadPersistentStores:

let description = NSPersistentStoreDescription()

description.shouldInferMappingModelAutomatically = true
description.shouldMigrateStoreAutomatically = true

container.persistentStoreDescriptions = [description]

Answer 2

In my case I make append to new description instead of replacement

let description = NSPersistentStoreDescription()
description.shouldInferMappingModelAutomatically = true
description.shouldMigrateStoreAutomatically = true
container.persistentStoreDescriptions.append(description)

Answer 3

With the new NSPersistentContainer there are many sensible defaults. You don't have to set the properties, which most of the answers give.

Simply create the container and load.

let container = NSPersistentContainer(name: dataModelName)
container.loadPersistentStores { storeDescription, error in
    // You may verify the default storeDescription has the right default for light migration
}

Answer 4

Note that both NSPersistentStore's shouldInferMappingModelAutomatically and shouldMigrateStoreAutomatically are true by default.

Apple reference for:

• shouldInferMappingModelAutomatically

• shouldMigrateStoreAutomatically

Answer 5

You can just start a new project in Xcode 7, and copy and paste the generated core data code to the new new project!

I created a project in Xcode 7 and migrated it to Xcode 8, this is the generated code. (I have already added the two options for enabling lightweight stack migration)

// MARK: - Core Data stack

lazy var applicationDocumentsDirectory: URL = {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "com.MyApp" in the application's documents Application Support directory.
    let urls = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
    return urls[urls.count-1]
}()

lazy var managedObjectModel: NSManagedObjectModel = {
    // The managed object model for the application. This property is not optional. It is a fatal error for the application not to be able to find and load its model.
    let modelURL = Bundle.main.url(forResource: "MyApp", withExtension: "momd")!
    return NSManagedObjectModel(contentsOf: modelURL)!
}()

lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator = {
    // The persistent store coordinator for the application. This implementation creates and returns a coordinator, having added the store for the application to it. This property is optional since there are legitimate error conditions that could cause the creation of the store to fail.
    // Create the coordinator and store
    let coordinator = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
    let url = self.applicationDocumentsDirectory.appendingPathComponent("SingleViewCoreData.sqlite")
    var failureReason = "There was an error creating or loading the application's saved data."
    do {
        try coordinator.addPersistentStore(ofType: NSSQLiteStoreType, configurationName: nil, at: url, options: [NSMigratePersistentStoresAutomaticallyOption: true,NSInferMappingModelAutomaticallyOption: true])
    } catch {
        // Report any error we got.
        var dict = [String: AnyObject]()
        dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data" as AnyObject?
        dict[NSLocalizedFailureReasonErrorKey] = failureReason as AnyObject?

        dict[NSUnderlyingErrorKey] = error as NSError
        let wrappedError = NSError(domain: "YOUR_ERROR_DOMAIN", code: 9999, userInfo: dict)
        // Replace this with code to handle the error appropriately.
        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
        NSLog("Unresolved error \(wrappedError), \(wrappedError.userInfo)")
        abort()
    }

    return coordinator
}()

lazy var managedObjectContext: NSManagedObjectContext = {
    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.) This property is optional since there are legitimate error conditions that could cause the creation of the context to fail.
    let coordinator = self.persistentStoreCoordinator
    var managedObjectContext = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
    managedObjectContext.persistentStoreCoordinator = coordinator
    return managedObjectContext
}()

// MARK: - Core Data Saving support

func saveContext () {
    if managedObjectContext.hasChanges {
        do {
            try managedObjectContext.save()
        } catch {
            // Replace this implementation with code to handle the error appropriately.
            // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
            let nserror = error as NSError
            NSLog("Unresolved error \(nserror), \(nserror.userInfo)")
            abort()
        }
    }
}

You can just copy the above and paste it in your App Delegate.

One thing to note is that please edit this line so that it gets your data model's URL:

let modelURL = Bundle.main.url(forResource: "MyApp", withExtension: "momd")!