How to Do a Lightweight CoreData Migration in Swift

Question

I am attempting my first lightweight CoreData migration. I read two guides on lightweight migrations. Both add code to a CoreDataStack class, modifying variables like NSPersistentStoreCoordinator and adding:

let mOptions = [NSMigratePersistentStoresAutomaticallyOption: true,
    NSInferMappingModelAutomaticallyOption: true]

My problem is that I have a perfectly functioning app using CoreData but I don't have that class or anything like it. My problem is, why do these projects assume I have this class and can I achieve my light weight migration without it? If not, how do I add it?

More Info, If Needed to Answer

In September I built an app using CoreData. It was my first time using CoreData and I followed this Ray Wenderlich guide. It worked great, I finished the app, and its now on the store. Now I'd like to start making some changes to the app, which involve new CoreData attributes and a few new entities. I've read that I need to setup a new model version.

I found a Ray Wenderlich guide but it uses this CoreDataStack.swift file that I don't have:

What's frustrating is that I setup CoreData using their guide and it didn't include that file! Then I go to do a migration and they assume I have it.

I went in search of another lightweight migration method, found this alternative and it too references code that I never built into my CoreData:

lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator? = {
// The persistent store coordinator for the application. This implementation creates and return 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
var coordinator: NSPersistentStoreCoordinator? = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("MyLog.sqlite")
var error: NSError? = nil
var failureReason = "There was an error creating or loading the application's saved data."
if coordinator!.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil, error: &error) == nil {
    coordinator = nil
    // Report any error we got.
    var dict = [String: AnyObject]()
    dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data"
    dict[NSLocalizedFailureReasonErrorKey] = failureReason
    dict[NSUnderlyingErrorKey] = error
    error = 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 \(error), \(error!.userInfo)")
    abort()
}

So I've read the guides and understand 90% of the tutorials. I just need someone to take a look at that original CoreData tutorial and tell me where, if I don't have a CoreData class, I would add the lightweight code such as:

let mOptions = [NSMigratePersistentStoresAutomaticallyOption: true,
    NSInferMappingModelAutomaticallyOption: true]

Mundi Mundi · Accepted Answer · 2016-01-20T23:55:58

The migration options need to be used in the call that added the persistent store to the persistent store coordinator. You will easily find this line of code by doing a search for addPersistentStoreWithType.

try coordinator!.addPersistentStoreWithType(
     NSSQLiteStoreType, configuration: nil, URL: url, options: mOptions)

Most likely your Core Data stack is in the AppDelegate class, but regardless where it is, this is where you have to add the migration options.

How to Do a Lightweight CoreData Migration in Swift

1 Answers