code: public function getTaggings()
      {
          if (is_null($this->taggings)) {
              foreach ($this as $key => $item) {
                  if (empty($item['tags'])) {
                      continue;
                  }
                  foreach ($item['tags'] as $tagging) {
                      if (!in_array($tagging, array_keys($item['tags']))) {
                          $this->taggings[$tagging] = array();
                      }
                      
                      $this->taggings[$tagging][] = $key;
                  }
              }
          }
          
          return $this->taggings;
      }
docs: Returns associative array of tags and the slugs of items
  with those tags
  i.e. 'tagfoo' => array('foo', 'bar')
  
  Caches the result so it does the loop once.