Custom migration

I'm trying to do a custom migration, changing an attribute name and type but it doesn't seem to be running the mapping. Old model has an attribute names "roundDrill" which is a Boolean. The new model has this removed and a new attribute called "drillType" which is a String. I have a mapping model called "V2V3.xcmappingmodel" where the entity has a custom policy set "Diamond_Painting_Logbok.V2V3". The new attribute has a value of "FUNCTION($entityPolicy, "roundToType:" , $source.roundDrill)" In the "V2V3.swift" I have the below code import UIKit import CoreData class V2V3: NSEntityMigrationPolicy { func roundToType(isRound:NSNumber) -> String { NSLog("Round: \(isRound)") if isRound.boolValue { return "Round" } else { return "Square" } } } When I print out the value of drillType, I'm getting nil. The NSLog on the function isn't being printed at all. If I change the custom policy name, its not giving me an error so appears as though its being ignored completely