thinkphp怎么插入主表和附表，当我们插入主表的时候，插入附表的id 是什么

public function add(){
if($_POST) {
if(!isset($_POST['title']) || !$_POST['title']) {
return show(0,'标题不存在');
}
if(!isset($_POST['small_title']) || !$_POST['small_title']) {
return show(0,'短标题不存在');
}
if(!isset($_POST['catid']) || !$_POST['catid']) {
return show(0,'文章栏目不存在');
}
if(!isset($_POST['keywords']) || !$_POST['keywords']) {
return show(0,'关键字不存在');
}
if(!isset($_POST['content']) || !$_POST['content']) {
return show(0,'content不存在');
}
if($_POST['news_id']) {
return $this->save($_POST);
}
$newsId = D("News")->insert($_POST);
if($newsId) {
$newsContentData['content'] = $_POST['content'];
$newsContentData['news_id'] = $newsId;
$cId = D("NewsContent")->insert($newsContentData);
if($cId){
return show(1,'新增成功');
}else{
return show(1,'主表插入成功，副表插入失败');
}

}else{
return show(0,'新增失败');
}

}else {

$webSiteMenu = D("Menu")->getBarMenus();

$titleFontColor = C("TITLE_FONT_COLOR");
$copyFrom = C("COPY_FROM");
$this->assign('webSiteMenu', $webSiteMenu);
$this->assign('titleFontColor', $titleFontColor);
$this->assign('copyfrom', $copyFrom);
$this->display();
}
}

$newsId = D("News")->insert($_POST);

$webSiteMenu = D("Menu")->getBarMenus();

$titleFontColor = C("TITLE_FONT_COLOR");
$copyFrom = C("COPY_FROM");
$this->assign('webSiteMenu', $webSiteMenu);
$this->assign('titleFontColor', $titleFontColor);
$this->assign('copyfrom', $copyFrom);
$this->display();