结合保存和文件上传的方法，文件是上传到服务器里面了，但是路径没有，保存到数据库只是一个空值。哪里出问题了？
public function upload(){
$model = M('model');
$num = $_GET['id']; //获取get变量
$condition['id']= $num;
$config= array(
'maxSize' =>3145728,
'exts' =>array('jpg', 'gif', 'png', 'jpeg'),
'replace' =>ture,
'rootPath' =>'./Public/span_img/',
// 'savePath' =>'Public/span_img/',
'autoSub' =>false,
'saveName' =>$num,
);
$upload = new \Think\Upload($config);// 实例化上传类
// 上传文件
$info = $upload->upload();
$data['img_url'] = $file['savepath'].$file['savename'];
$result = $model->where($condition)->save($data);
if(!$info) {// 上传错误提示错误信息
$this->error($upload->getError());
}else{// 上传成功
$this->success('上传成功！');
}
}