I believe there's been an api change with the latest release.
Basically add_field() member function changed to add_filter()

So does "Any module performing a search" imply that your module is implementing hook_search?

It would be really, really nice to have a link to any documentation on the interface to the $query object referred to here (Drupal_Solr_Query_Interface). I spent an afternoon searching and wound up reading the module source code.

Edited to add another 'really'. Still searching days later. What's in the query? What's in params? Why are my added filters lost when I set aliases? Is it a bug or am I using it wrong? Heck if I know. It has to be written down somewhere.

After developing some code for a website that we were building that required the integration of CCK types that were of type integer I thought I would post the following code to help others out.

CCK fields alter (CCK strings deal with these types out of the box but not for integers tragically)

function yourmodule_apachesolr_cck_fields_alter(&$mappings) {
  $mappings['number_integer'] = array(
   'optionwidgets_select' => array(
      // This function is called when teh facet is being indexed
      'indexing_callback' => 'yourmodule_cck_checklist_indexing_callback',
      // This function will get called when the facet links are displayed
      // in facet blocks at search time.
      'display_callback' => 'yourmodule_cck_checklist_display_callback',
     'index_type' => 'integer'),
   'optionwidgets_buttons' => array(
      'indexing_callback' => 'yourmodule_cck_checklist_indexing_callback',
      // This function will get called when the facet links are displayed
      // in facet blocks at search time.
      'display_callback' => 'olesolr_cck_checklist_display_callback',
      'index_type' => 'integer'),
  );
  return $mappings;
}

Next function renders the checklist block

function olesolr_cck_checklist_indexing_callback($node, $field_name, $cck_info) {
  // $fields is an array because we send back a key => value pair for every
  // value of the field.
  $fields = array();

  // Don't do anything if this node doesn't have this field.
  if (isset($node->$field_name)) {
    // Get the index key based on the $cck_info.
    $index_key = apachesolr_index_key($cck_info);
    //dsm($index_key);

    // Go through each item and setup the appropriate key
    foreach ($node->$field_name as $field) {
      foreach($field as $key => $value) {
        if ($index_value =  (isset($value) ? $value : FALSE)) {
          $fields[] = array(
            'key' => $index_key,
            'value' => $index_value,
          );
        }
      }
    }
  }
  return $fields;
}

Finally the display callback (which is pretty basic)

function yourmodule_cck_checklist_display_callback($facet, $options) {
  if (function_exists('content_format')) {
    return content_format($options['delta'], array('value' => $facet));
  }
  else {
    return $facet;
  }
}

I've moved from faceted_search to apachesolr and missed the faceted_search navigation blocks. Apachesolr only shows it's blocks on the search pages (search/apachesolr). This is a quick hack to perform a custom search and show the corresponding taxonomy facete block. (apachesolr 6.x-2.x-dev)

/* dirty hack to make apachesolr show the facetes block
*  it's hardcoded to initiate apachesolr search on a certain page (custom view at /store)
*  see in action here http://4soldo.ru/store (left sidebar)
*/

function forcesolr_init() {

	switch(arg(0)) {
		// see if we're on the right page
	    case 'store' :
		
		// get apachesolr query instance
		// function apachesolr_drupal_query($keys = '', $filters = '', $solrsort = '', $base_path = '')
		// don't forget the $browse_path
		$query = apachesolr_drupal_query('','','','search/apachsolr_search');
		// query params
		$params = array(
		'start' => 0,
		'rows' => 0,
		'facet' => 'true', // this one is important
		);
		// add all configured facetes to $params
		apachesolr_search_add_facet_params($params, $query);

		// add type filter (I only need 'store' content type)
		// 'fsl' doesn't mean anything or used anywhere else,
		// just like apachesolr_fsl below
		$params['fq']['fsl'][] = 'type:store';
		
		// once again, this time for blocks
		// without this blocks don't remember the filter
		$query->add_filter('type','store');		

		// This hook allows modules to modify the query and params objects.
		// Actually we need this to process $params['fq']['fsl'][]
		apachesolr_modify_query($query, $params, 'apachesolr_fsl');
		if (empty($query)) {
		  return;
		}

		// get solr object				
		$solr = apachesolr_get_solr();
	
		$result = $solr->search($query, 0, 0, $params);
		if(empty($result)) return;
		
		// time to check what we got from Solr
#		drupal_set_message('<pre>result' . print_r($result,true) . '</pre>');
		
		// get rid of unneeded 'type' facete (it's already set to 'store')
		// also I had to hide 'current search' block via block visibility options
		unset($result->facet_counts->facet_fields->type);
		
		// save the solr response so it can be used by blocks lates
		apachesolr_static_response_cache($result,'apachesolr_search');
		// cache the query info
		apachesolr_current_query($query);
		// rise the flag
		apachesolr_has_searched(TRUE);
	
	}

}

Here's the live page using this code: http://4soldo.ru/store (in russian)
The same result can be acheived with apachesolr_views, but it's for views 3 only.
Hope it helps someone.

I am dealing with the same issue as MA-Hugh...