gistfile1.m

NSMutableDictionary *mutableAttributeValues = [attributeValues mutableCopy];
  			
		NSString *resourceIdentifier = AFResourceIdentifierFromReferenceObject([self referenceObjectForObjectID:objectID]);
		
		[childContext performBlock:^{
			[backingContext performBlockAndWait:^{
				NSManagedObjectID *backingObjectID = [self objectIDForBackingObjectForEntity:entity withResourceIdentifier:resourceIdentifier];
				NSManagedObject *backingObject = [backingContext existingObjectWithID:backingObjectID error:nil];
				[backingObject setValuesForKeysWithDictionary:mutableAttributeValues];
				
				NSString *lastModified = [[operation.response allHeaderFields] valueForKey:@"Last-Modified"];
				if (lastModified) {
					[backingObject setValue:lastModified forKey:kAFIncrementalStoreLastModifiedAttributeName];
				}
				
				NSString *etag = [[operation.response allHeaderFields] valueForKey:@"Etag"];
				if (etag) {
					[backingObject setValue:etag forKey:kAFIncrementalStoreEtagAttributeName];
				}
				
				AFSaveManagedObjectContextOrThrowInternalConsistencyException(backingContext);
			}];
			
			
			[childContext performBlock:^{
				NSManagedObject *managedObject = [childContext objectWithID:objectID];
				
				[mutableAttributeValues addEntriesFromDictionary:attributes];
				[mutableAttributeValues removeObjectForKey:kAFIncrementalStoreLastModifiedAttributeName];
				[mutableAttributeValues removeObjectForKey:kAFIncrementalStoreEtagAttributeName];
				[managedObject setValuesForKeysWithDictionary:mutableAttributeValues];
				
				AFSaveManagedObjectContextOrThrowInternalConsistencyException(childContext);
				[self notifyManagedObjectContext:context aboutRequestOperation:operation forNewValuesForObjectWithID:objectID];
			}];
		}];