Custom rewrite for product variation not working

Question

I'm trying to rewrite the product variation url for a Phone Model attribute for example to this url:

index.php?product=example&attribute_pa_model=iphone-x

This works when I open it in the browser directly. So the original url I'd like to be:

/product/example/iphone-x

I tried with the code below, but it is not working.

function add_model_taxonomy_args($args) {
    $args['query_var'] = 'attribute_pa_model';
    return $args;
}
add_filter('woocommerce_taxonomy_args_pa_model', 'add_model_taxonomy_args' ); 

function custom_rewrite_rules() {
    add_rewrite_tag('%attribute_pa_model%', '([a-zA-Z0-9-]+)');
    add_rewrite_rule('^product/(.+?)/(.+?)/?$', 'index.php?product=$matches[1]&attribute_pa_model=$matches[2]', 'top');
}

Answer 1

This works because on this URL, $_REQUEST['attribute_pa_model'] is set, and in this example, the value is iphone-x:

index.php?product=example&attribute_pa_model=iphone-x

But on this URL, $_REQUEST['attribute_pa_model'] is not set, so the auto-selection of the product variation does not work:

/product/example/iphone-x

So on that URL, you can use the woocommerce_dropdown_variation_attribute_options_args to filter the selected value, like so:

add_filter( 'woocommerce_dropdown_variation_attribute_options_args', 'auto_select_attribute_pa_model' );
function auto_select_attribute_pa_model( $args ) {
    // If it's not the pa_model taxonomy, don't filter the $args.
    if ( empty( $args['selected'] ) && 'pa_model' === $args['attribute'] ) {
        $args['selected'] = get_query_var( 'attribute_pa_model' );
    }

    return $args;
}

Additional Note

In my tests, this is not necessary and can be removed:

add_filter('woocommerce_taxonomy_args_pa_model', 'add_model_taxonomy_args' );