Redirect to first child on Custom Post Type (without template)

Question

I need to redirect to the first child page in a hierarchical Custom Post Type. For the love of me I have scoured high and low, but all the solutions seem to involve applying a template to the parent that kicks off redirection.

I need to bypass the template and redirect to the first child with a nice, clean bit of code in functions.php. Can someone help?

Thanks!

Answer 1

I'm not sure how you define "first child" - alphabetically, by highest or lowest ID, etc., but if you can query for the child you want via WP_Query, then you can filter single_template to do the redirection based on whether the requested post has a parent or not:

function wpa87313_redirect_post_type( $template ){
    // if this isn't the right post type, return
    if( ! is_singular( 'my_post_type' ) )
        return $template;

    // if this post has a parent, return
    global $wp_query;
    if( 0 != $wp_query->post->post_parent )
        return $template;

    // query for the child of this parent
    // here's where you need to sort out what a first child is  
    $child = new WP_Query(
        array(
            'post_parent' => $wp_query->post->ID,
            'post_type'   => 'my_post_type',
            'posts_per_page' => 1,
            // orderby?
            // order?
        )
    );

    // if a child was found, redirect to it
    if( ! empty( $child->posts ) ){
        wp_redirect( get_permalink( $child->post->ID ) );
        exit;
    }

    return $template;
}

add_filter( 'single_template', 'wpa87313_redirect_post_type', 10, 1 );