Cluster.php

    // Create a finder for clusters not linked to 'object'
    static::finder('unlinked', function($self, $params, $chain) {
      $object = $params['options']['object'];
      unset($params['options']['object']);

      // @todo Shouldn't make a direct call to getClusters, but should instead 
      // have a protected array of things that can be linked
      $ids = $object->getClusters()->map(function($cluster) {
        return $cluster->id;
      }, array('collect' => false));

      if (count($ids) > 0) {
        $params['options']['conditions'] = array(
          'id' => array(
            '!=' => $ids
          )
        );
      }

      return $chain->next($self, $params, $chain);
    });