code: public static function getOne(Item $parent, Item $child, $options = [])
      {
          $key = md5($parent->systemId . "." . $child->systemId);
          if (isset(self::$_relationships[$key])) {
              self::$_relationships[$key]->mergeOptions($options);
          } else {
              self::$_relationships[$key] = new Relationship($parent, $child, $options);
          }
  
          return self::$_relationships[$key];
      }
docs: Get one.
  
  @param cascade\components\types\Item $parent  [[@doctodo param_description:parent]]
  @param cascade\components\types\Item $child   [[@doctodo param_description:child]]
  @param array                         $options [[@doctodo param_description:options]] [optional]
  
  @return [[@doctodo return_type:getOne]] [[@doctodo return_description:getOne]]