Versioning and Migration - Core Data - page 142

import CoreData
import UIKit
class AttachmentToImageAttachmentMigrationPolicyV3toV4:
NSEntityMigrationPolicy {
}
This naming convention should look familiar to you, as it's noting that this is a
custom migration policy and is for transforming data from Attachments in model
version 3 to ImageAttachments in model version 4.
Back in the v3-to-v4 mapping model file, select the
AttachmentToImageAttachment entity mapping. In the Entity Mapping
Inspector, fill in the Custom Policy field with the class name you just created,
UnCloudNotes.AttachmentToImageAttachmentMigrationPolicyV3toV4 .
Now, when Core Data runs this migration, it will create an instance of your custom
migration policy when it needs to perform a data migration for that specific set of
data. That's your chance to run any custom transformation code to do what's
needed to extract image information during migration! Now, it's time to add some
custom logic to the custom entity mapping policy.
Open AttachmentToImageAttachmentMigrationPolicyV3toV4.swift and add
the method to perform the migration:
override func createDestinationInstancesForSourceInstance(
sInstance: NSManagedObject !,
entityMapping mapping: NSEntityMapping !,
manager: NSMigrationManager !, error: NSErrorPointer ) -> Bool {
// 1
let newAttachment =
NSEntityDescription . insertNewObjectForEntityForName (
"ImageAttachment" ,
inManagedObjectContext: manager. destinationContext )
as NSManagedObject
// 2
for propertyMapping in mapping. attributeMappings
as [ NSPropertyMapping ]! {
let destinationName = propertyMapping. name
if let valueExpression = propertyMapping. valueExpression {
let context: NSMutableDictionary = [ "source" : sInstance]
let destinationValue: AnyObject =
valueExpression. expressionValueWithObject (sInstance,
context: context )