AppDelegate.m

RKObjectManager* objectManager = [RKObjectManager objectManagerWithBaseURLString:@"http://mysite.com/"];

// Core Data store
RKManagedObjectStore* objectStore = [RKManagedObjectStore objectStoreWithStoreFilename:@"RKData.sqlite"];
objectManager.objectStore = objectStore; 

// Activity indicator
objectManager.client.requestQueue.showsNetworkActivityIndicatorWhenBusy = YES;

// MAPPING - Model (detail item)
RKManagedObjectMapping* modelMapping = [RKManagedObjectMapping mappingForEntityWithName:@"Model" inManagedObjectStore:objectManager.objectStore];

modelMapping.primaryKeyAttribute = @"modelID"; 
[modelMapping mapKeyPath:@"id" toAttribute:@"modelID"];
[modelMapping mapKeyPath:@"title" toAttribute:@"title"];
[modelMapping mapKeyPath:@"image" toAttribute:@"image"];

[objectManager.mappingProvider registerMapping:modelMapping withRootKeyPath:@"model"];


[objectManager.mappingProvider setObjectMapping:modelMapping forResourcePathPattern:@"json.php" withFetchRequestBlock:^NSFetchRequest *(NSString *resourcePath){

    return [Model fetchRequest];
}];

// MAPPING - Category (Master)
RKManagedObjectMapping* categoryMapping = [RKManagedObjectMapping mappingForEntityWithName:@"Category" inManagedObjectStore:objectManager.objectStore];

categoryMapping.primaryKeyAttribute = @"categoryID";  
[categoryMapping mapKeyPath:@"id" toAttribute:@"categoryID"];
[categoryMapping mapKeyPath:@"title" toAttribute:@"title"];
[categoryMapping mapKeyPath:@"image" toAttribute:@"image"];
[categoryMapping mapRelationship:@"models" withMapping:modelMapping];

[objectManager.mappingProvider registerMapping:categoryMapping withRootKeyPath:@"category"];

[objectManager.mappingProvider setObjectMapping:categoryMapping forResourcePathPattern:@"json.php" withFetchRequestBlock:^NSFetchRequest *(NSString *resourcePath){
    
    return [Category fetchRequest];
}];

myModels.json

[{"category":
	{"id":2,"title":"Your Models",
		"models":[
			{"id":23,"title":"Cowboy","image":"Cowboy.png"},
			{"id":24,"title":"Lizard","image":"Lizard.png"}
			]
	}
}]

ViewController.m

- (void)viewDidLoad
{
    [super viewDidLoad];
    [self loadDetailItem];  
}

- (void)loadDetailItem {
    
    RKObjectManager* objectManager = [RKObjectManager sharedManager];
    [objectManager loadObjectsAtResourcePath:@"json.php" delegate:self];
    
}

- (void)objectLoader:(RKObjectLoader *)objectLoader didLoadObjects:(NSArray *)objects {
    
    NSFetchRequest* fetchRequest = [Category fetchRequest];
    self.categories = [[Category objectsWithFetchRequest:fetchRequest] retain] ;
    
    [self.tableView reloadData]; 
}

Did you get it working?

No, not working yet.