def level_for_item(navi_key)
      return level if self[navi_key]

      items.each do |item|
        next unless item.sub_navigation
        level = item.sub_navigation.level_for_item(navi_key)
        return level if level
      end
      return nil
    end
def level_for_item(navi_key)
      return level if self[navi_key]

      items.each do |item|
        next unless item.sub_navigation
        level = item.sub_navigation.level_for_item(navi_key)
        return level if level
      end
      return nil
    end
Returns the level of the item specified by navi_key.
 Recursively works its way down the item's sub_navigations if the desired
 item is not found directly in this container's items.
 Returns nil if item cannot be found.