How to have a hierarchy of custom post types and use two of them in the permalink?

Question

General Setup

I'm working on a setup to create a documentary/wiki site for a company. The site should have three different levels of hierarchy and one type of content at the lower end:

• level 1: area (high level structure, e.g. 'internal stuff' or 'external projects')
• level 2: topic (main structure, e.g. 'controlling' or 'project xyz')
• level 3: section (groups the content, for a project this could be 'project management')
• level 4: content (custom post type that holds the content, basically like pages)

Issue 1

Ideally I have a custom post type for all four level. But is this even possible? Can I have a custom post type under another custom post type? I didn't find anything about this...

Permalinks

So the permalinks should be like this:

mysite.com/area/external-projects

Simply lists all topics in this area, so area is kind of a category for topic

mysite.com/topic/project-xyz

Lists all the sections and the content entries.

Link to section is not needed.

The link to content is tricky and the point where I need help:

mysite.com/topic/my-topic/content/this-is-my-content

Issue 2

So, the permalinks for the content should include the topic and the content. Is this even possible if the section (the hierarchy between them) is not in the permalink?

Answer 1

You can do this with custom hierarchical taxonomy and use it for your custom post type.

Register the Taxonomy area for the post type content using the init action hook:

function wporg_register_taxonomy_area() {
   $labels = array(
     'name'              => _x( 'Areas', 'taxonomy general name' ),
     'singular_name'     => _x( 'Area', 'taxonomy singular name' ),
     'search_items'      => __( 'Search Areas' ),
     'all_items'         => __( 'All Areas' ),
     'parent_item'       => __( 'Parent Area' ),
     'parent_item_colon' => __( 'Parent Area:' ),
     'edit_item'         => __( 'Edit Area' ),
     'update_item'       => __( 'Update Area' ),
     'add_new_item'      => __( 'Add New Area' ),
     'new_item_name'     => __( 'New Area Name' ),
     'menu_name'         => __( 'Area' ),
   );
   $args   = array(
     'hierarchical'      => true, // make it hierarchical (like categories)
     'labels'            => $labels,
     'show_ui'           => true,
     'show_admin_column' => true,
     'query_var'         => true,
     'rewrite'           => [ 'slug' => 'area' ],
   );
   register_taxonomy( 'area', [ 'content' ], $args );
}
add_action( 'init', 'wporg_register_taxonomy_area' );

Register custom post type content using the init action hook:

function my_custom_content_item() {
    $labels = array(
        'name'               => _x( 'Content', 'post type general name' ),
        'singular_name'      => _x( 'Content', 'post type singular name' ),
        'menu_name'          => 'Content'
    );
    $args = array(
        'labels'        => $labels,
        'public'        => true,
        'has_archive'   => true,
        'taxonomies'    => array( 'area' ),
        'show_in_rest'  => true,
    );
    register_post_type( 'content', $args );

} 
add_action( 'init', 'my_custom_content_item' );

Adds the registered taxonomy area to object type content.

function attach_area_to_content() {
    register_taxonomy_for_object_type( 'area', 'content' );
}
add_action('init','attach_area_to_content');