This solved my problems. Its adding another field to index and you can create facet out of it.
function wtc_glossary_search_api_alter_callback_info() {
$callbacks['wtc_glossary_alter_add_first_letter_title'] = array(
'name' => t('First letter of listing title'),
'description' => t("This module provides first letter of title for glossary view."),
'class' => 'WtcAlterAddFirstLetter',
);
return $callbacks;
}
/**
* Search API data alteration callback that adds the first letter of title for glossary mode
*/
class WtcAlterAddFirstLetter extends SearchApiAbstractAlterCallback {
public function alterItems(array &$items) {
foreach ($items as $id => &$item) {
if (!isset($item->FIELD_YOU_NEED)) {
$item->search_api_title_first_letter = NULL;
continue;
}
$item->search_api_title_first_letter = substr($item->FIELD_YOU_NEED,0,1);
}
}
public function propertyInfo() {
return array(
'search_api_title_first_letter' => array(
'label' => t('First Letter of FIELD_YOU_NEED'),
'description' => t('For for listings in glossary mode.'),
'type' => 'text',
),
);
}
}