//数据
$id=2
$data['cname'] = 'aa';
$data['news_content']['content']="aaaaabbbb";

//$res = $News->relation(true)->where("id=$id")->save($data); //这种方法关联更新失败，出错的sql是：SQL: UPDATE `think_news_content` SET `content`='xxxx123' WHERE news_id=''

$data['id']=2;
$res = $News->relation(true)->save($data);//这种方法更新成功