Creating facets for Apache SOLR Multisite Search

This page shows how to create new facets for the SOLR Multisite search module.

This issue lists the steps I went through and are more like notes to self than real documentation.

Standard text facets

You first need to patch the module as described in the issue. I could post a patch, but pasting the code yourself guarantees you'll be able to apply the correction if the module evolves.

It basically renders the apachesolr_multisitesearch_enabled_facets_form function hookable.

You can then implement this in your module :

function yourmodule_apachesolr_multisitesearch_facets($rebuild = FALSE) {
  $facets = array();
  $facets['yourfacetfield'] = array(
    'info' => t('yourmodule search: Filter by human_readable_name_of_your_field'),
    'facet_field' => 'yourfacetfield',
  );
  return $facets;
}

To enable a block for your facet, you have to implement hook_block :

function yourmodule_block($op = 'list', $delta = 0, $edit = array()) {
  switch ($op) {
    case 'list':
      // This code is copied from the original apachesolr_multisitesearch module
      $enabled_facets = apachesolr_multisitesearch_enabled_facets('yourmodule');
      // This line must be modified to take advantage of the new hook
      $facets = yourmodule_apachesolr_multisitesearch_facets();
      // Add the blocks
      $blocks = array();
      foreach ($enabled_facets as $delta => $facet_field) {
        if (isset($facets[$delta])) {
          $blocks[$delta] = $facets[$delta] + array('cache' => BLOCK_CACHE_PER_PAGE,);
        }
      }
      return $blocks;
      break;
     
    case 'view' :
      if (apachesolr_multisitesearch_has_searched()) {
        // Get the query and response. Without these no blocks make sense.
        $response = apachesolr_static_response_cache();
        if (empty($response)) {
            return;
        }
        $query = apachesolr_current_query();

        // Here is the place where you inform Drupal about your blocks
        switch ($delta) {
          case 'yourfacetfield' :
            return apachesolr_facet_block($response, $query, 'yourmodule', $delta, $delta, t('Filter by human_readable_name_of_your_field'));
        }
      }
  }
}

Your facet should then be visible on the page admin/settings/apachesolr/multisite-filters.

Once you’ve enabled it, you should see a corresponding block on admin/build/block/list.

You can customize the block a bit if you like, and then assign it to a region.

You don’t have to worry about visibility settings as the various Drupal SOLR modules take care of this for you.

For a standard text facet, that’s all there is to it.

Numeric values based facets

If you need to build a facet based on a numeric field and want some understandable (mean : string) output, you have to code a callback function. Otherwise, numeric values will be shown which seldomly make sense for the user.

In the code for hook_block above :

case 'yourfacetfield' :
                        return apachesolr_facet_block($response, $query, 'yourmodule', $delta, $delta, t('Filter by human_readable_name_of_your_field'));

You must add a call to your callback function :

case 'yourfacetfield' :
                        return apachesolr_facet_block($response, $query, 'yourmodule', $delta, $delta, t('Filter by human_readable_name_of_your_field'), 'yourmodule_yourfacetfield');

You can name the function whatever you want, but a bit of namespacing never hurts.

You then have to code your callback function:

function yourmodule_yourfacetfield($facet) {
    if (! $facet) {
        return "No value";   
    }
   // Fetch whatever text value matching the numeric id passed by the facet
   // Here, as often, we just query the node database table
   // but you can code any way to match your ids to string values
   $sql = 'SELECT title FROM {node} WHERE nid = %d';
   return db_result(db_query($sql, $facet));
}

Other considerations

• Beware if you personalized some fields in schema.xml. Some parsers render custom string fields unusable for faceting. With the standard type ‘string’, no problem.
• It’s a bit out of this page's scope, but anytime you change something in schema.xml, you have to restart the SOLR service, or the container running it (i.e /etc/init.d/tomcat restart or something like that) otherwise your changes are not validated.
• You can test your environment directly from the SOLR interface: http://yoursite.com/solr/select/?q=testtext&fl=*,score&sort=field_to_sor... desc&facet=true&facet.field=yourfacetfield. It can really be useful to test this way before spending hours wondering what's wrong with your code.

I used it to enable more than 10 facets on one of my sites. Feel free to comment.