我一直在尝试退回一美元wpdb->；插入\\u id；但不幸的是，它正在返回null；我做错了什么？我使用ajax从前端发送数据。这是后端代码。

function add_fav() {

    global $wpdb;
    $table = $wpdb->prefix . "fav_products";
    $productID = (int) $_POST[\'product_id\'];
    $response = []; 
    $insertid = 3;
    if(isset($_POST[\'amount\'])) {
        $amount = (int) $_POST[\'amount\'];
    }
    else{
        $amount = 1;
    }
    
    if(isset($_COOKIE[\'fav_id\']) && !empty($_COOKIE[\'fav_id\']))
    {
        $fav_id = $_COOKIE[\'fav_id\'];
    }
    else
    {
        
        $fav_id = uniqid();
        setcookie(\'fav_id\', $fav_id, strtotime(\'+1 year\'), \'/\');
    }
      
    $itemInfav = $wpdb->get_row($wpdb->prepare("SELECT id, fav_id, product_id FROM $table WHERE fav_id = %s AND product_id = %d", $fav_id, $productID));
        
    if(!empty($itemInfav->product_id))
    {
        
        $response[\'sorry\'] = "Sorry the product already exist in Wishlist!";
    
    }
    else
    {
        // insert 
        $toevoegen = $wpdb->insert( 
            $table, 
            array( 
                \'fav_id\' => $fav_id, 
                \'product_id\' => $productID,
                \'amount\' => $amount,
            ), 
            array( 
                \'%s\', 
                \'%d\',
                \'%d\' 
            ) 
        );
        $lastinsertid = $toevoegen->insert_id;
    }
    
    $totalInfav = $wpdb->get_row($wpdb->prepare("SELECT SUM(AMOUNT) as total FROM $table WHERE fav_id = %s", $fav_id));
    $response[\'result\'] = $toevoegen;
    $response[\'fav_id\'] = $fav_id;
    $response[\'total\'] = $totalInfav->total;
    $response[\'insertid\'] = $lastinsertid;
    wp_send_json( $response );
    wp_die();
}

下面是我的数据库结构。我确实有外键在里面。

CREATE TABLE $favtable (
   id int(11) NOT NULL AUTO_INCREMENT,
   product_id int(11) NOT NULL,
   fav_id varchar(255) NOT NULL,
   amount int(11) NOT NULL,
   created_at timestamp NOT NULL DEFAULT current_timestamp(),
   PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

这就是前端ajax的外观。

$(\'body\').on(\'click\', \'.addToFav\', function(e)
    {
        e.preventDefault();
        
        var product_id = $(this).data(\'product-id\'),
        ajax = $(this).data(\'ajax\'),
        siteurl = $(this).data(\'url\'),
        uid = $(this).data(\'uid\'),
        $image = $(\'.\'+uid).children(\'img\'),
        img = document.createElement("img");
        img.src = siteurl+\'/wp-content/themes/bg/assets/images/favorite-icon-white.svg\',
        
        $.post(ajax, { 
            action: \'add_fav\', 
            product_id: product_id}, 
            function(data){
                console.log(data);
                updateFavIcon(parseInt(data.total));    

                if( data.result ) { 
                    console.log(data.insertid); 
                    $(\'.\'+uid).removeClass(\'addToFav\');
                    $(\'.\'+uid).addClass(\'addedFav\');
                    $image.remove();
                    $(\'.\'+uid).append(img);
                    $(\'.\'+uid).data(\'wid\', data.insertid);
                    $(\'.\'+uid).attr(\'data-wid\', data.insertid);
                    
                }
            });
    });

谁能帮我找出我做错了什么，最后一个插入id没有返回？提前谢谢。

        // insert 
        $toevoegen = $wpdb->insert( 
            ...
        );
        $lastinsertid = $toevoegen->insert_id;