NSCoder implementations for preserving NSManagedObject ObjectIDs as URLs and restoring those objects from a managed object context.

ACodableManagedObject.mm

@implementation ACodableManagedObject

+ (id)ObjectWithManagedObject:(NSManagedObject*)obj
{
   ACodableManagedObject* codableObj = nil;
   if (nil != obj)
   {
      codableObj = [[ACodableManagedObject alloc] init];
      codableObj.url = obj.objectID.URIRepresentation;
   }
   return codableObj;
}

- (id)initWithCoder:(NSCoder*)aDecoder
{
   self = [super init];
   if (nil != self)
   {
      self.url = [aDecoder decodeObjectForKey:@"url"];
   }
   return self;
}

- (void)encodeWithCoder:(NSCoder*)aCoder
{
   [aCoder encodeObject:self.url forKey:@"url"];
}

@end

AManagedObjectArchiver.mm

@implementation AManagedObjectArchiver

+ (NSData*)ArchiveDataFromBlock:(void(^)(AManagedObjectArchiver* archiver))codingBlock
{
   NSMutableData* data = [NSMutableData data];
   AManagedObjectArchiver* archiver =
   [[AManagedObjectArchiver alloc] initForWritingWithMutableData:data];

   codingBlock(archiver);

   [archiver finishEncoding];

   return data;
}

- (void)encodeObject:(id)obj forKey:(NSString*)key
{
   if ([obj isKindOfClass:[NSManagedObject class]])
   {
      ACodableManagedObject* codableObj =
      [ACodableManagedObject ObjectWithManagedObject:obj];

      [self encodeObject:codableObj forKey:key];
   }
   else
   {
      [super encodeObject:obj forKey:key];
   }
}

@end

AManagedObjectUnarchiver.mm

+ (void)UnarchiveData:(NSData*)data
              Context:(NSManagedObjectContext*)context
                Block:(void(^)(AManagedObjectUnarchiver* unarchiver))decodingBlock
{
   AManagedObjectUnarchiver* unarchiver = [[AManagedObjectUnarchiver alloc] initForReadingWithData:data];
   unarchiver.context = context;

   decodingBlock(unarchiver);

   [unarchiver finishDecoding];
}

- (id)decodeObjectForKey:(NSString*)key
{
   id obj = [super decodeObjectForKey:key];

   if ([obj isKindOfClass:[ACodableManagedObject class]])
   {
      ACodableManagedObject* codableObj = obj;
      NSURL* url = codableObj.url;

      NSManagedObjectID* objectId =
      [self.context.persistentStoreCoordinator
       managedObjectIDForURIRepresentation:url];
      
      obj = [self.context existingObjectWithID:objectId error:NULL];
   }

   return obj;
}

@end