Drupal 6: Creating a custom Lucene Search facet from taxonomy and integrating into the search form block

In this tutorial I'll show how you to create a custom Lucene Search facet from taxonomy and integrate into the search form block. A great first step would be to review the Drupal Lucene API documentation (see luceneapi.api.php PHP file in the luceneapi module folder).

I assigned the search form block to a region in my theme (admin/build/block), which generates this form:

Search form block

I added a new vocabulary called "Topics", and added a few terms (admin/content/taxonomy).

Topic and terms

The goal of this code is to integrate the Topic vocabulary into the search form block to allow the user to select a taxonomy term as they search. To start, you need to implement a hook_luceneapi_facet_realm() and a callback function in your custom module.

 * Implements hook_luceneapi_facet_realm()
function MYMODULE_luceneapi_facet_realm() {

  $realms = array();

  $realms['form'] = array(
    'title' => t('Search form block'),
    'callback' => 'MYMODULE_luceneapi_facet_realm_callback_search_form_block',
    'callback arguments' => array(),
    'allow empty' => TRUE,
    'description' => t('Displays facets in the search form block.'),

  return $realms;


 * Implements hook_luceneapi_facet_realm() callback function
function MYMODULE_luceneapi_facet_realm_callback_search_form_block($facets, $realm, $module) {

  $form = array();

  // loop through facets
  foreach ($facets as $name => $facet) {

    // NOTE: luceneapi_facet_to_fapi_convert() converts a Lucene facet to Drupal Form API data
    $form = array_merge_recursive($form, luceneapi_facet_to_fapi_convert($facet));


  return $form;


At this point if you go to the facets admin page (admin/settings/luceneapi_node/facets), you can see the newly created realm. I assigned the taxonomy vocabulary "Topic" to this realm.

Assigning facets to realms

Up next is implementing a hook_form_alter() to integrate the facet into the search form.

 * Implementation of hook_form_FORM_ID_alter().
function MYMODULE_form_search_block_form_alter(&$form, &$form_state) {

  // get default search module (IE: luceneapi_node)
  $module = luceneapi_setting_get('default_search');

  // check if default search module is defined in lucene searchable module list
  if (array_key_exists($module, luceneapi_searchable_module_list())) {

    // get index type (IE: node)
    $type = luceneapi_index_type_get($module);

    // fetch realm facets
    $elements = luceneapi_facet_realm_render('form', $module, $type);

    // if facet form elements exist, recursively merge with current form object
    if (!empty($elements)) {
      $form = array_merge_recursive($form, $elements);



The search from block should now show an empty "Topic" facet.

Search form block empty topic

The last piece of code implements hook_luceneapi_facet_postrender_alter() which gives you the opportunity to modify the facet, and in the this case, add its options.

Immediately after implementing this hook, if you krumo() or dsm() the $items argument, you'll see the form element has no options.

Using krumo to see empty form element

The next section of code copied the contrib module code in "Lucene Node". [See file: luceneapi/contrib/luceneapi_node/luceneapi_node.module; function: function luceneapi_node_luceneapi_facet_postrender_alter()]

 * Implements hook_luceneapi_facet_postrender_alter()
function MYMODULE_luceneapi_facet_postrender_alter(&$items, $realm, $module, $type = NULL) {

  if ($realm == 'form' && $module == 'luceneapi_node' && $type == 'node' && is_array($items['category'])) {

    // get taxonomy form data
    $taxonomy = module_invoke('taxonomy', 'form_all', 1);

    // get enabled facets
    $facets_enabled = luceneapi_facet_enabled_facets_get($module, $realm);

    // loop through enabled facets, validate, and fetch weight
    $weights = array();
    foreach ($facets_enabled as $name => $value) {

      // check for "category" facet
      // FORMAT: category_{VOCABID}
      if (preg_match('/^category_(\d+)$/', $name, $match)) {

        // load taxonomy vocabulary
        if ($vocabulary = taxonomy_vocabulary_load($match[1])) {

          // ensure category and vocab id is enabled for this module and realm
          if (luceneapi_facet_enabled($match[0], $module, 'form')) {

            // fetch weight
            $variable = sprintf('luceneapi_facet:%s:%s:%s:weight', $module, $realm, $name);
            $weights[$vocabulary->name] = variable_get($variable, 0);




    // end foreach

    // gets weighted taxonomy array
    $taxonomy_weighted = array();
    foreach ($weights as $vocab_name => $weight) {
      $taxonomy_weighted[$vocab_name] = $taxonomy[$vocab_name];

    // create array of fapi data to override
    $category_data = array(
      '#prefix' => '<div class="criterion">',
      '#suffix' => '</div>',
      //'#size' => 10,
      '#options' => $taxonomy_weighted,
      '#multiple' => TRUE,
      '#default_value' => luceneapi_facet_value_get('category', array()),
      '#title' => NULL,
      '#description' => NULL,

    // merge data
    $items['category'] = array_merge($items['category'], $category_data);

    // sets weight as the lowest weight of all taxonomy facets
    if (is_array($items['category']['#weight'])) {
      $items['category']['#weight'] = min($items['category']['#weight']);

  // end if


Reloading the page will now show the search form with a completed facet.

Search form with facet

Submitting the search form block with a selected taxonomy term will now take the user to the search results page with the taxonomy facet pre-selected!