Restkit 0.20 pre4 core data issue - core-data

Question

{"status":"ok",
"count":5,
"count_total":165,
"pages":33,
"posts":[{ "id":2971,
"status":"publish",
"title":"title1",
"content":"",
"date":"date",
"categories":[{"id":5,
"title":"category1"},
{"id":7,
"title":"category2"}],
"thumbnail":"url",
"custom_fields":{"wpcf-content":["content"],
"wpcf-audio":["url"]}
},
{ "id":2974,
"status":"publish",
"title":"title2",
"content":"",
"date":"date2",
"categories":[{"id":5,
"title":"category1"},
{"id":5,
"title":"category3"}
],
"thumbnail":"url",
"custom_fields":{"wpcf-content":["content"],
"wpcf-audio":["url"]}
}
]
}
posts.h
#interface posts : NSManagedObject
#property (nonatomic, retain) NSString* title;
#property (nonatomic, retain) NSArray* wpcf-content;
#property (nonatomic, retain) NSArray* wpcf-audio;
#property (nonatomic, retain) NSString* id;
#property (nonatomic, retain) NSArray* categories;
#end
categories.h
#interface categories : NSManagedObject
#property (nonatomic, retain) NSString* title;
#end
Model.xcdatamodeld
entities Posts : title (string), id (integer 64),wpcf-audio (transformable),wpcf-content (transformable)
Categories : title (string)
NSManagedObjectModel *managedObjectModel = [NSManagedObjectModel mergedModelFromBundles:nil];
RKManagedObjectStore *managedObjectStore = [[RKManagedObjectStore alloc] initWithManagedObjectModel:managedObjectModel];
NSString *path = [RKApplicationDataDirectory() stringByAppendingPathComponent:#"Model.sqlite"];
[managedObjectStore addSQLitePersistentStoreAtPath:path fromSeedDatabaseAtPath:nil withConfiguration:nil options:nil error:nil];
[managedObjectStore createManagedObjectContexts];
RKEntityMapping *categoryMapping = [RKEntityMapping mappingForEntityForName:#"Categories" inManagedObjectStore:managedObjectStore];
[categoryMapping addAttributeMappingsFromDictionary:#{ #"title":#"title"}];
RKEntityMapping *mapping = [RKEntityMapping mappingForEntityForName:#"Posts" inManagedObjectStore:managedObjectStore];
NSArray *identification =[[NSArray alloc]initWithObjects:#"id", nil];
mapping.identificationAttributes= identification;
[mapping addAttributeMappingsFromArray:#[#"title",#"id",#"custom_fields.wpcf-content",#"custom_fields.wpcf-audio"]];
NSIndexSet *statusCodes = RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful); // Anything in 2xx
RKResponseDescriptor *responseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:mapping pathPattern:nil keyPath:#"posts" statusCodes:statusCodes];
[mapping addPropertyMapping:[RKRelationshipMapping relationshipMappingFromKeyPath:#"categories" toKeyPath:#"categories" withMapping:categoryMapping]];
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:#"http://my.url/json"]];
RKManagedObjectRequestOperation *operation = [[RKManagedObjectRequestOperation alloc] initWithRequest:request responseDescriptors:#[responseDescriptor]];
operation.managedObjectContext = managedObjectStore.mainQueueManagedObjectContext;
[operation setCompletionBlockWithSuccess:^(RKObjectRequestOperation *operation, RKMappingResult *result) {
posts *article = [result firstObject];
categories *catego =[article.categories objectAtIndex:0];
NSLog(#"Mapped the article: %#", article.title);
NSLog(#"Mapped the article: %#", catego.title);
} failure:^(RKObjectRequestOperation *operation, NSError *error) {
NSLog(#"Failed with error: %#", [error localizedDescription]);
}];
RKObjectManager *manager = [RKObjectManager managerWithBaseURL:[NSURL URLWithString:#"http://my.url/"]];
[manager enqueueObjectRequestOperation:operation];
I get this error W restkit.core_data:RKManagedObjectMappingOperationDataSource.m:157 Performing managed object mapping with a nil managed object cache:
Unable to update existing object instances by identification attributes. Duplicate objects may be created.