How to set parent-child relationship between differents custom post types

I have just set up a post/parent relationship between a post type “episodes” and a post type “cartoon-series.”

I used this bit of code to add in the meta box to assign the parent from another post type:

add_action('admin_menu', function() {
    remove_meta_box('pageparentdiv', 'episodes', 'normal');
});
add_action('add_meta_boxes', function() {
    add_meta_box('episodes-parent', 'Cartoon Series', 'episodes_attributes_meta_box', 'episodes', 'side', 'default');
});

function episodes_attributes_meta_box($post) {
    $post_type_object = get_post_type_object($post->post_type);
    if ( $post_type_object->hierarchical ) {
        $pages = wp_dropdown_pages(array('post_type' => 'cartoon-series', 'selected' => $post->post_parent, 'name' => 'parent_id', 'show_option_none' => __('(no parent)'), 'sort_column'=> 'menu_order, post_title', 'echo' => 0));
        if ( ! empty($pages) ) {
            echo $pages;
        } // end empty pages check
    } // end hierarchical check.
}

That worked on the admin screen in allowing me to set the series as a parent to the episode, but when I try to view the post, I get a 404. The url structure is:

domain/episodes/series-name/episode-name

The url for the series is:

domain/cartoon-series/series-name

I’d like the url for the episode to be:

domain/cartoon-series/series-name/episode-name

What am I missing? Is it possible to make an entire post type the child of another post type? So, then I could even get the url for the episodes list to be:

domain/cartoon-series/series-name/episodes

Thanks!
Matt

As requested, here is the code for the two custom post types in question:

$labels = array(
    "name" => "Cartoon Series",
    "singular_name" => "Cartoon Series",
    "menu_name" => "Cartoon Series",
    "all_items" => "All Cartoon Series",
    "add_new" => "Add New",
    "add_new_item" => "Add New Cartoon Series",
    "edit" => "Edit",
    "edit_item" => "Edit Cartoon Series",
    "new_item" => "New Cartoon Series",
    "view" => "View",
    "view_item" => "View Cartoon Series",
    "search_items" => "Search Cartoon Series",
    "not_found" => "No Cartoon Series Found",
    "not_found_in_trash" => "No Cartoon Series Found in Trash",
    "parent" => "Parent Cartoon Series",
    );

$args = array(
    "labels" => $labels,
    "description" => "",
    "public" => true,
    "show_ui" => true,
    "has_archive" => true,
    "show_in_menu" => true,
    "exclude_from_search" => false,
    "capability_type" => "post",
    "map_meta_cap" => true,
    "hierarchical" => true,
    "rewrite" => array( "slug" => "cartoon-series", "with_front" => true ),
    "query_var" => true,
    "supports" => array( "title", "revisions", "thumbnail" ),           );
register_post_type( "cartoon-series", $args );

$labels = array(
    "name" => "Episodes",
    "singular_name" => "Episode",
    );

$args = array(
    "labels" => $labels,
    "description" => "",
    "public" => true,
    "show_ui" => true,
    "has_archive" => true,
    "show_in_menu" => true,
    "exclude_from_search" => false,
    "capability_type" => "post",
    "map_meta_cap" => true,
    "hierarchical" => true,
    "rewrite" => array( "slug" => "episodes", "with_front" => true ),
    "query_var" => true,
    "supports" => array( "title", "revisions", "thumbnail" ),           );
register_post_type( "episodes", $args );

I’m using the CPT UI plugin, so I can’t edit that code directly. That is just the export code CPT UI provides.

I don’t have any other code that links the two CPTs. Maybe that’s what I’m missing. I just found that code online that places the metabox on the page to do the linking. Is it not enough to do the job? Looks like it sets the post_parent.

Thanks!
Matt

Finally I’ve found a working solution. The cartoon-series can be registered as you did but episodes custom post types can not be hirarchical (I think WordPress expects parent content be the same type as child content if the relationship is set using post_parent in wp_posts database table).

When registering episodes, the rewrite rule must be set to the slug you want, that is cartoon-series/%series_name%. Then we can filter the episodes link to replace %series_name% with actual name of the parent cartoon-series post type and a rewrite rule to say to WordPress when a cartoon-series post type is requested and when is a episodes.

add_action('init', function(){
    $labels = array(
        "name" => "Cartoon Series",
        "singular_name" => "Cartoon Series",
        "menu_name" => "Cartoon Series",
        "all_items" => "All Cartoon Series",
        "add_new" => "Add New",
        "add_new_item" => "Add New Cartoon Series",
        "edit" => "Edit",
        "edit_item" => "Edit Cartoon Series",
        "new_item" => "New Cartoon Series",
        "view" => "View",
        "view_item" => "View Cartoon Series",
        "search_items" => "Search Cartoon Series",
        "not_found" => "No Cartoon Series Found",
        "not_found_in_trash" => "No Cartoon Series Found in Trash",
        "parent" => "Parent Cartoon Series",
    );

    $args = array(
        "labels" => $labels,
         "description" => "",
        "public" => true,
        "show_ui" => true,
        "has_archive" => true,
        "show_in_menu" => true,
        "exclude_from_search" => false,
        "capability_type" => "post",
        "map_meta_cap" => true,
        "hierarchical" => true,
        "rewrite" => array( "slug" => "cartoon-series", "with_front" => true ),
        "query_var" => true,
        "supports" => array( "title", "revisions", "thumbnail" )
    );

    register_post_type( "cartoon-series", $args );

    $labels = array(
        "name" => "Episodes",
        "singular_name" => "Episode",
    );

    $args = array(
        "labels" => $labels,
        "description" => "",
        "public" => true,
        "show_ui" => true,
        "has_archive" => true,
        "show_in_menu" => true,
        "exclude_from_search" => false,
        "capability_type" => "post",
        "map_meta_cap" => true,
        "hierarchical" => false,
        "rewrite" => array( "slug" => "cartoon-series/%series_name%", "with_front" => true ),
        "query_var" => true,
        "supports" => array( "title", "revisions", "thumbnail" )
    );

    register_post_type( "episodes", $args );

});

add_action('add_meta_boxes', function() {
    add_meta_box('episodes-parent', 'Cartoon Series', 'episodes_attributes_meta_box', 'episodes', 'side', 'default');
});

function episodes_attributes_meta_box($post) {
        $pages = wp_dropdown_pages(array('post_type' => 'cartoon-series', 'selected' => $post->post_parent, 'name' => 'parent_id', 'show_option_none' => __('(no parent)'), 'sort_column'=> 'menu_order, post_title', 'echo' => 0));
        if ( ! empty($pages) ) {
            echo $pages;
        } // end empty pages check
}

add_action( 'init', function() {

    add_rewrite_rule( '^cartoon-series/(.*)/([^/]+)/?$','index.php?episodes=$matches[2]','top' );

});

add_filter( 'post_type_link', function( $link, $post ) {
    if ( 'episodes' == get_post_type( $post ) ) {
        //Lets go to get the parent cartoon-series name
        if( $post->post_parent ) {
            $parent = get_post( $post->post_parent );
            if( !empty($parent->post_name) ) {
                return str_replace( '%series_name%', $parent->post_name, $link );
            }
        } else {
            //This seems to not work. It is intented to build pretty permalinks
            //when episodes has not parent, but it seems that it would need
            //additional rewrite rules
            //return str_replace( '/%series_name%', '', $link );
        }

    }
    return $link;
}, 10, 2 );

NOTE: Remember to flush rewrite rules after saving the above code and before to try it. Go to wp-admin/options-permalink.php and click save to regerenate the rewrite rules.

NOTE 2: It is probably that more rewrite rules have to be added, for example to work for paginate posts. Also it may need some more work in order to have a complete solution, for example, when deleting a cartoon-series delete also all child episodes? Add a filter in admin edit screen to filter episodes by post parent? Modify the episodes title in admin edit screen to show the parent series name?