next_prev.rb

def next
  page = self.class.choose(self, 'rgt >', :first)
  page ||= self.class.choose(self, 'lft <', :first)
end

def prev
  page = self.class.choose(self, 'lft <', :last)
  page ||= self.class.choose(self, 'rgt >', :last)
end


class << self

  def choose(page, position_condition, pick)
    position_method = position_condition.split.first

    results = where("#{position_condition} ? AND depth = ? AND parent_id = ?", 
                    page.send(position_method.to_sym), 
                    page.depth, 
                    page.parent_id)
                .live
                .in_menu
                .order(position_method) # ASC is default
                
    results.send(pick)
  end
end