How to parse a JSON string using RestKit, without loading a remote resource

gistfile1.m

NSString *jsonString = [self readJSON:@"some_file"];
NSDictionary *dictionary = [jsonString objectFromJSONString];

RKObjectMapper *mapper = [RKObjectMapper mapperWithObject:dictionary 
                                          mappingProvider:[MappingFactory sharedMappingProvider]];
RKObjectMappingResult *result = [mapper performMapping];

// MappingFactory
+ (RKObjectMappingProvider *)sharedMappingProvider
{
	if (! mappingProvider) {
		mappingProvider = [RKObjectMappingProvider new];
		[mappingProvider setMapping:[self createFirstMapping] forKeyPath:@"firstKey"];
		[mappingProvider setMapping:[self createSecondMapping] forKeyPath:@"secondKey"];
	}
	return mappingProvider;
}