thinkphp实现上一篇与下一篇的方法

//上一篇 
$front=$Article->where("id<".$id)->order('id desc')->limit('1')->find(); 
$this->assign('front',$front); 
//下一篇 
$after=$Article->where("id>".$id)->order('id desc')->limit('1')->find(); 
$this->assign('after',$after);

//上一篇 
$front=$Classroom->where("nid<".$nid)->order('nid desc')->limit('1')->find(); 
$f=!$front?'没有了':'.__URL__.'/nid/'.$front['nid'].'">'.$front['title'].''; 
$this->assign('front',$f); 
//下一篇 
$after=$Classroom->where("nid>".$nid)->order('nid desc')->limit('1')->find(); 
$a=!$after?'没有了':'.__URL__.'/nid/'.$after['nid'].'">'.$after['title'].''; 
$this->assign('after',$a);