Custom Post Type Tag Capabilities Not Working

Question

I'm on WordPress 4.7.4 and on my custom post types, I cannot assign tags as a regular user.

I have used CPTUI for creating everything, have mapped and created custom capabilities, have assigned roles to the custom caps, and have verified the user's roles are correct using get_userdata(), but I can't seem to get this working.

On the CPT, the taxonomies variable has post_tag set and I can see the tags meta box, I just can't do anything with it. I have tried to separate the term capabilities using the function below and the user has the assign_post_tags role, but it doesn't seem to do anything. Any help would be greatly appreciated.

Register CPT:

function cptui_register_my_cpts_listing() {

    /**
     * Post Type: Listings.
     */

    $labels = array(
        "name" => __( 'Listings', 'text-domain' ),
        "singular_name" => __( 'Listing', 'text-domain' ),
    );

    $args = array(
        "label" => __( 'Listings', 'text-domain' ),
        "labels" => $labels,
        "description" => "",
        "public" => true,
        "publicly_queryable" => true,
        "show_ui" => true,
        "show_in_rest" => false,
        "rest_base" => "",
        "has_archive" => "archive-listing",
        "show_in_menu" => true,
        "exclude_from_search" => false,
        "capability_type" => "listing",
        "map_meta_cap" => true,
        "hierarchical" => false,
        "rewrite" => array( "slug" => "listing", "with_front" => true ),
        "query_var" => "listing",
        "supports" => array( "title", "editor", "thumbnail" ),
        "taxonomies" => array( "post_tag", "location" ),
    );

    register_post_type( "listing", $args );
}

add_action( 'init', 'cptui_register_my_cpts_listing' );

Changing caps

function set_builtin_tax_caps() {
    $tax = get_taxonomy('post_tag');
    $tax->cap->manage_terms = 'manage_post_tags';
    $tax->cap->edit_terms = 'edit_post_tags';
    $tax->cap->delete_terms = 'delete_post_tags';
    $tax->cap->assign_terms = 'assign_post_tags';
    $tax = get_taxonomy('category');
    $tax->cap->manage_terms = 'manage_categories';
    $tax->cap->edit_terms = 'edit_categories';
    $tax->cap->delete_terms = 'delete_categories';
    $tax->cap->assign_terms = 'assign_categories';
}
add_action('init', 'set_builtin_tax_caps');

Answer 1

It seems that although WP core creates four capability mappings for the built-in post_tag taxonomy (manage_terms => manage_post_tags, edit_terms => edit_post_tags, delete_terms => delete_post_tags, assign_terms => assign_post_tags) when it is registered, it uses different values when checking if the user has one of those capabilities.

If you look at the implementation of the map_meta_cap function in WP core (in wp-includes/capabilities.php lines 513-523), you can see that the edit_posts capability is returned for assign_post_tags and manage_categories is returned for manage_post_tags, edit_post_tags and delete_post_tags. To fix this, you can add a filter and return the expected values:

add_filter( 'map_meta_cap', function( $caps, $cap, $user_id, $args ) {
    $caps_to_fix = [
        'manage_post_tags',
        'edit_post_tags',
        'delete_post_tags',
        'assign_post_tags',
    ];

    if ( in_array( $cap, $caps_to_fix ) ) {
        $caps = [ $cap ];
    }

    return $caps;
}, 10, 4 );

Answer 2

No matter what I tried, I couldn't get this working. I don't believe manually changing the terms are supported at this time for the post_tag taxonomy. Even if the user had the proper role and permission, you could not assign or edit post tags. The only solution I found was to create and register an entirely new taxonomy called "tags", define the custom capabilities, and set hierarchical to false.