add_rewrite_rule only works when flush first

Question

I have the following code in my functions.php file. I have a page projects. I have two custom post types: project and projecttype. An individual project post has it's own permalink /projects/project-name. Projecttypes do need to show the 'projects' page.

My code checks if the url is a subpage of /projects/. If the subpage is a projecttype, it loads the template for the projects page (id 8).

if(substr( $_SERVER['REQUEST_URI'], 0, 11 ) === '/projects/'){
  $request_subpage = str_replace('/', '', str_replace('/projects/', '', $_SERVER['REQUEST_URI']));
  if($request_subpage !== '') {
    $valid_subpage_url = false;
    $project_types = get_posts( array( 'post_type' => 'projecttype', 'posts_per_page' => -1 ) );
    foreach( $project_types as $project_type ) :
      if( $project_type->post_name === $request_subpage ) :
        $valid_subpage_url = true;
        break;
      endif;
    endforeach;
    //flush_rewrite_rules();
    if($valid_subpage_url) add_rewrite_rule( "^projects\/$request_subpage$", "index.php?page_id=8", 'top');

  }
}

When I uncomment flush_rewrite_rules(); my code works. When I don't flush the rewrite rules, it does not work (all projecttype subpages return a 404). Since flush is an expensive operation, I probably shouldn't use it.

What's going on here?

Answer 1

Problem with rules occurs, because rule for a given projecttype is added when you visit relevant URL.

Every time you visit one of projects/{projectstype}/ addresses and flush rewrite rules, they are deleted and only one rule (only for current project type) is saved/created.

You can add your code to request filter hook, then there is no need to add rewrite rules. All query vars, that are set by rules (you set only page_id), you can set manually.

add_filter( 'request' , 'se336968_request' );
function se336968_request( $query_vars )
{
    if(substr( $_SERVER['REQUEST_URI'], 0, 11 ) === '/projects/'){
        $request_subpage = str_replace('/', '', str_replace('/projects/', '', $_SERVER['REQUEST_URI']));
        if($request_subpage !== '') {
            $project_types = get_posts( array( 'post_type' => 'projecttype', 'posts_per_page' => -1 ) );
            foreach( $project_types as $project_type ) {
                if( $project_type->post_name === $request_subpage ) {

                          // your current rewrite 
                    $query_vars['page_id'] = 8;
                    $query_vars['post_type'] = 'page';

                    //  --- to open "projecttype" type post from URL ---
                    // $query_vars['post_type'] = 'projecttype';
                    // $query_vars['projecttype'] = $request_subpage;
                    // unset( $query_vars['page_id'], $query_vars['p'], $query_vars['pagename'], $query_vars['name'] );

                    break;
                }
            }
        }
    }
    return $query_vars;
}