How to update Post Meta values through the comment system

I’m working on allowing users to rate posts – through the comment meta. Storing and displaying individual ratings is working, however, I would like to be able to store the average rating in the post meta.

I have this thus far, seem to get stuck on passing values to the post meta.

add_action( 'comment_post', 'add_av_rating', 1 );
function add_av_rating($comment_ID)
{           
    $commentdata=get_comment($comment_ID, ARRAY_A); 
    $parent_post=get_post($commentdata['comment_post_ID']);
    //get rating, get post meta & calc 
update_post_meta($parent_post, 'average_wr', $commentdata);
}

Even when I put a value such as ‘thisText’ instead of $commentdata it seems to fail. What am I doing wrong?