wordpress: Automatically add category to post when certain other category is selected

gistfile1.php

<?php 
function add_category_automatically1($post_ID) {
		global $wpdb;
			if(in_category(7)){
				$cat = array(10);
				wp_set_object_terms($post_ID, $cat, 'category', true);
			}
}
add_action('publish_post', 'add_category_automatically1');