RestKit and saving to CoreData as NSManagedObject

I'm using RestKit and i want to parse and save elements to core data. I have two json files:

First (Category):

[
   {
     "cat_id": 3371,
     "cat_name": "myName",
     "image": 762
   },
   {
     "cat_id": 3367,
     "cat_name": "anotherName",
     "image": 617
   }
]

And second (Elements):

[
  {
    "art_id": "1",
    "node": {
      "author": "name"
    },
    "small_url": 0
  },
  {
    "art_id": "12",
    "node": {
      "author": "anotherName"
    },
    "small_url": 0
  }
]

So the basic idea is that every category have some elements inside. So this is my CoreData struct:

I've download the restkit example and use TwitterCoreData sample. My code is: AppDelegeta.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    NSURL *baseURL = [NSURL URLWithString:@"http://globalURL.com"];
    RKObjectManager *objectManager = [RKObjectManager managerWithBaseURL:baseURL];
    NSManagedObjectModel *managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];
    RKManagedObjectStore *managedObjectStore = [[RKManagedObjectStore alloc] initWithManagedObjectModel:managedObjectModel];
    objectManager.managedObjectStore = managedObjectStore;

    RKEntityMapping *categoryMapping = [RKEntityMapping mappingForEntityForName:@"Category" inManagedObjectStore:managedObjectStore];
    categoryMapping.identificationAttributes = @[ @"catId" ];
    [categoryMapping addAttributeMappingsFromDictionary:@{
     @"cat_id": @"catId",
     @"node.author": @"author",
     }];

    RKEntityMapping *elementsMapping = [RKEntityMapping mappingForEntityForName:@"Elements" inManagedObjectStore:managedObjectStore];
    elementsMapping.identificationAttributes = @[ @"artId