Сергей Буравцов

user_id=1 - тоже параметр, он должен быть в params, но не в pattern/path

RKObjectMapping *profilesMapping = [RKObjectMapping mappingForClass:[LVKUser class]];
    [profilesMapping addAttributeMappingsFromArray:@[@"uid",
                                                     @"first_name",
                                                     @"last_name",
                                                     @"nickname"]];
    
    RKResponseDescriptor *responseDescriptor =
    [RKResponseDescriptor responseDescriptorWithMapping:profilesMapping
                                                 method:RKRequestMethodGET
                                            pathPattern:@"users.get"
                                                keyPath:@"response"
                                            statusCodes:[NSIndexSet indexSetWithIndex:200]];
    
    [objectManager addResponseDescriptor:responseDescriptor];
    
    NSDictionary *params = @{@"fields": @"nickname", @"user_id": @"1"};
    
    [[RKObjectManager sharedManager] getObjectsAtPath:@"users.get"
                                           parameters:params
                                              success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult){
                                                  NSLog(@"%@", mappingResult.array);
                                              }
                                              failure:^(RKObjectRequestOperation *operation, NSError *error){
                                                  NSLog(@"%@", error);
                                                  NSLog(@":c");
                                              }
     ];