Custom entity migration in swift coredata not working - ios

Question

I changed the type of one of the fields in an entity i created in iOS from a dictionary to a list and created a new version for that entity which is set as default but kept the same name, but migration is not working as whenever i try to iterate that field in a deserialized object it appears to still be a dictionary due to unrecognized selector error for indexAt i receive.
I created a subclass of NSEntityMigrationPolicy to migrate a field from a dictionary to string type ([String : TimerData] -> [TimerData]) with a method like so:
class SessionTransformationPolicy : NSEntityMigrationPolicy {
#objc func transformTimerRows(data: [String : TimerData]?) -> [TimerData] {
return Array(data!.values).map { orig in
TimerData(old: orig)
}
}
}
I created a mapping model and set it to this migration policy, but whenever i put breakpoints or print statements in the migration policy it doesnt get executed, i made sure to specify in the model mapping the method i created based on online sources:
FUNCTION($entityPolicy, "transformTimerRowsWithData:" , $source.timerRows)
I've done a bunch of research into this issue and the only solution i've found was to go into the app delegate and change shouldInferMappingModelAutomatically for the container to false but this just results in no data being loaded and the migration policy still not being called, can someone tell me what i'm doing wrong?
lazy var persistentContainer: NSPersistentContainer = {
let container = NSPersistentContainer(name: "Model")
let description = NSPersistentStoreDescription()
description.shouldMigrateStoreAutomatically = true
description.shouldInferMappingModelAutomatically = false
container.persistentStoreDescriptions = [description]
container.loadPersistentStores(completionHandler: { (storeDescription, error) in
if let error = error as NSError? {
fatalError("Unresolved error \(error), \(error.userInfo)")
}
else {
}
})
return container
}()