How to catch Rewrite rules then display a specific post?

Question

I'm trying to catch the params sent by the rewrite rules and then display a selected post.

But for now, even if I can retrieve those param, the page redirect to the archive of my custom post type. Where I need to display the post that match a custom query build from the query string given by the rewrite rule.

add_action('init', function () {
    $terms_slug = getCustomTermsSlug();
    // @note Catch /{foo}
    add_rewrite_rule('^(' . implode('|', $terms_slug) . ')/?', 'index.php?post_type=custom&foo=$matches[1]', 'top');
});

add_action('init', function () {
    add_rewrite_tag('%foo%', '([^&]+)');
}, 10, 0);

add_action('pre_get_posts', function ($query) {
    if (is_admin() || !$query->is_main_query()) {
        return;
    }

    if (get_query_var('foo')) {
        $tax_query = [[
            'taxonomy' => 'custom',
            'field' => 'slug',
            // @note "foo" is a custom taxonomy term
            'terms' => get_query_var('foo'),
        ]];

        $query->set('tax_query', $tax_query);
    }

    return $query;
});

First, I don't understand why it redirect to the archive page?

Second, maybe the "pre_get_posts" is not the right place to make the customized query but where I can do it then?

Third, how to display the selected post in response of the clean URL given?

Thank's for your help! ;-)

@EDIT I've tried using actions like "template_redirect" or "parse_request" but it feel really *hacky. Is anyone know how to force the redirect (eg. rewrite matching) to the first post of the custom query?

Answer 1

It appear, like @milo also said, that the correct way to doing this is by using the "parse_request" action that stand before the "pre_get_posts" action (see https://codex.wordpress.org/Plugin_API/Action_Reference).

The complete solution I came with :

add_action('init', function () {
    $refs_slug = getCustomRefTermsSlug(); // this will return an array of all the terms slug existing in the custom "ref" taxonomy

    // @note Catch /{foo|bar|baz} and redirect to index.php?catch_me=true?ref={foo|bar|baz}
    add_rewrite_rule('^(' . implode('|', $refs_slug) . ')/?', 'index.php?post_type=custom&catch_me=true&ref=$matches[1]', 'top');
});

add_action('init', function () {
    add_rewrite_tag('%ref%', '([^&]+)');
    add_rewrite_tag('%catch_me%', '([^&]+)');
}, 10, 0);

add_action('parse_request', function ($query) {
    if (array_key_exists('catch_me', $query->query_vars) && $query->query_vars['catch_me']) {
        $args = [
            'post_type' => 'custom',
            'posts_per_page' => 1,
            'fields' => 'ids',
            'tax_query' => [],
        ];

        if (array_key_exists('ref', $query->query_vars)) {
            $args['tax_query'][] = [
                'taxonomy' => 'ref',
                'field' => 'slug',
                'terms' => $query->query_vars['ref'],
            ];
        }

        // get the right post
        $tmp_query = new \WP_Query($args);
        $post_id = $tmp_query->posts[0];

        // tell wordpress what post to display
        $query->query_vars = [
            'post_type' => 'custom',
            'p' => $post_id,
        ];
    }

    return $query;
});

add_filter('post_type_link', function ($url, $post) {
    if (get_post_type($post) == 'custom') {
        $refs_slug = getCustomRefTermsSlug($post->ID); // this will return an array of all the terms slug of the custom "ref" taxonomy assigned to the post

        // @note Return "/{foo|bar|bar}" as the permalink of the post
        return '/' . implode('/', $refs_slug);
    }

    return $url;
}, 10, 2);

Feel free to give me comments! ;-)