Sorting by node reference cck field

HI,
I am newbabie to drupal.

In my current project, I need to add custom sorting element.
I have three multivalued node reference cck fields which I need to add in custom sort.

Using hook_apachesolr_prepare_query, I have added cck field in apachesolr sorting array. But when I click on any sorting, then It did not work.

/* query function to add sorting elements */

function contract_search_apachesolr_prepare_query(&$query, &$params) {
$query->set_available_sort(
'im_field_cmp_se_service', array(
'title' => t('Service'),
'default' => 'asc',
));

$query-> set_available_sort( 'im_field_cmp_se_regions', array('title'=>t('Regions'),'default'=>'asc')
);

}

Kindly suggest me how can I add my multivalued cck fields to apachesolr sorting array and make it work.

Thanks in advance