onethink中的一个数据库联结问题

onethink的Admin/Article/getDocumentList（）方法；
我直接把代码粘上来了

protected function getDocumentList($cate_id=0,$model_id=null,$position=null,$field=true,$group_id=null){
        /* 查询条件初始化 */
        $map = array();
        if(isset($_GET['title'])){
            $map['title']  = array('like', '%'.(string)I('title').'%');
        }
        if(isset($_GET['status'])){
            $map['status'] = I('status');
            $status = $map['status'];
        }else{
            $status = null;
            $map['status'] = array('in', '0,1,2');
        }
        if ( isset($_GET['start']) ) {
            $map['update_time'][] = array('egt',strtotime(I('start')));
        }
        if ( isset($_GET['end']) ) {
            $map['update_time'][] = array('elt',24*60*60 + strtotime(I('end')));
        }
        if ( isset($_GET['nickname']) ) {
            $map['uid'] = M('Member')->where(array('nickname'=>I('nickname')))->getField('uid');
        }
        if(!is_administrator(UID)){
            $map["uid"] = UID;
        }
        // 构建列表数据
        $Document = M('Document');

        if($cate_id){
            $map['category_id'] =   $cate_id;
        }
        $map['pid']         =   I('pid',0);
        if($map['pid']){ // 子文档列表忽略分类
            unset($map['category_id']);
        }
        $Document->alias('DOCUMENT');
        if(!is_null($model_id)){
            $map['model_id']    =   $model_id;
            if(is_array($field) && array_diff($Document->getDbFields(),$field)){
                $modelName  =   M('Model')->getFieldById($model_id,'name');
                $Document->join('__DOCUMENT_'.strtoupper($modelName).'__ '.$modelName.' ON DOCUMENT.id='.$modelName.'.id');
                $key = array_search('id',$field);
                if(false  !== $key){
                    unset($field[$key]);
                    $field[] = 'DOCUMENT.id';
                }
            }            
        }
        if(!is_null($position)){
            $map[] = "position & {$position} = {$position}";
        }
        if(!is_null($group_id)){
            $map['group_id']    =    $group_id;
        }
        $list = $this->lists($Document,$map,'level DESC,DOCUMENT.id DESC',$field);

        if($map['pid']){
            // 获取上级文档
            $article    =   $Document->field('id,title,type')->find($map['pid']);
            $this->assign('article',$article);
        }
        //检查该分类是否允许发布内容
        $allow_publish  =   get_category($cate_id, 'allow_publish');

        $this->assign('status', $status);
        $this->assign('allow',  $allow_publish);
        $this->assign('pid',    $map['pid']);

        $this->meta_title = '文档列表';
        return $list;
    }

其中

$Document->join('__DOCUMENT_'.strtoupper($modelName).'__ '.$modelName.' ON DOCUMENT.id='.$modelName.'.id');

这段联结看不懂啊，有人能帮我解释一下吗，我知道'__DOCUMENT_'.strtoupper($modelName)代表另一个模型。'__ '.$modelName.'这个就不知道有什么用了。

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起别名 相当于AS