Add custom URL segment to wordpress page - php

Question

I'm trying to allow my page to receive query variable but to rewrite it to nice permalink. So I wanted this
example.com/wordpress-page/random
So I don't want random to be a subpage or something like that, since it's coming from outside service. In order to do this, I've added this code to my functions.php
function add_my_var($public_query_vars) {
$public_query_vars[] = 'my_var';
return $public_query_vars;
}
add_filter('query_vars', 'add_my_var');
function do_rewrite() {
add_rewrite_rule('wordpress-page/([^/]+)/?$', 'index.php?name=wordpress-page&my_var=$matches[1]','top');
}
add_action('init', 'do_rewrite');
When I go to example.com/wordpress-page I get my page, but when I go to example.com/wordpress-page/random I get 404 page.
I have flushed rewrite rules by saving permalinks in wp-admin panel.
Where did I go wrong?

Answer 1

I found I needed to call the flush_rewrite_rules() function. That was done in addition to the query_vars action hook and add_rewrite_rule() function you already demonstrated above.
In my case I chose to put it in my theme's functions.php using the switch_theme action hook and deactivated/reactivated my theme. But you can probably put it elsewhere as long as it get's called once at some point (so probably not 'init')
add_action('switch_theme', 'mytheme_setup_options');
function mytheme_setup_options () {
flush_rewrite_rules();
}