add custom field to parent post while adding attachment

I hooked to add_attachment, and I want to add custom field to the parent post with the URL of the new attachment.

I am using this code:

function twp_attachment_handler($attach_ID)
{
	$attach_post_obj = get_post($attach_ID);

	// check if post is not a post, than return
	$parent_post_obj = get_post($attach_post_obj->post_parent);
	if ($parent_post_obj->post_type != 'post')
		return;

	$url = wp_get_attachment_url($attach_ID);

	add_post_meta($attach_post_obj->post_parent, 'full', $url);

	return;
}

The problem is that if upload before I save for the first time, the custom field is not added. It’s as if the post_parent field is not initialized yet.

Anything I can do about it? Or maybe I should hook somewhere else?

BTW, I am using the new v2.3

Thanks