save_post how to find post_id?

Hi,

I’m writing here because I have a stupid problem and somehow I can’t find a solution on the internet.

So here’s the thing, I have a rating system I’d like to set up on some posts when I publish them.
Basically, the idea is to initiate their presence in the new table I set up in my database.

I ran a test in the front end, my function works

function register_rating(){
	global $post, $wpdb;

	$vote = 0;
	$total = 0;

		$query_verif = 'SELECT * FROM wp_ratings WHERE post_id = '. $post->ID;

		$r = $wpdb->get_results($query_verif);
		if (empty($r)){
			$query_reg = 'INSERT INTO wp_ratings (key_rating, post_id, nb_vote, total) VALUES ("",'. $post->ID.','. $vote.','. $total.')';
			$wpdb->query($query_reg);

		}

}

It does create the new field in the database.
Now, for some reason of proper working and not saturating the front-end with loads of queries, I’d like this to happen when I save the post in the admin panel.

so I hooked it:

add_action( 'save_post', 'register_rating' );

but…this doesn’t work. Nothing is happening.

For testing purposes, I changed the hook on ‘admin_menu’, so I could display the query. It turns out that there is no id after the ‘=’.

What am I doing wrong? How do I get this to work? There is so few info on the internet on this matter of getting the postid in the save_post…

Thanks a lot for your help

Mac