Custom Taxonomy Invalid in REST API

Question

I have a Custom API End Point to query data from another blog in WPMU, the custom end point loads and I am able to return some data, but the specific query I need returns 0 posts.

I have 131 posts in this custom post type and 11 of which are associated with the custom taxonomy.

I can query all 131 posts by using this.

switch_to_blog(1);

$args = array(
    'post_type' => 'podcast',
    'posts_per_page' => -1,
);
$query = new WP_Query( $args );
$posts = $query->posts;

restore_current_blog();

return $posts // returns 131 posts of custom post type

However, when I apply the custom taxonomy query I get 0 results

switch_to_blog(1);

$args = array(
    'post_type' => 'podcast',
    'posts_per_page' => -1,
    'tax_query' => array(
        array(
            'taxonomy' => 'theme',
            'field'    => 'slug',
            'terms'    => 'my-slug',
        ),
    ),
);
$query = new WP_Query( $args );
$posts = $query->posts; 

restore_current_blog();

return $posts; // returns 0 posts

I then tried to get terms of the custom post type to verify and I get invalid taxonomy. It appears my custom taxonomy isn't being registered within the REST API.

switch_to_blog(1)

$terms = get_terms( array(
    'taxonomy' => 'themes',
) );
restor_to_current_blog();

return $terms;

The output of which is

code    "invalid_taxonomy"
message "Invalid taxonomy."
data    null

I also tried to query the custom taxonomy without the switch_to_blog function and again got invalid taxonomy.

I also moved the registration of my custom post type from the functions.php to a mu-plugin thinking it could be an issue of using different themes for each of my sub blogs. But the results were still invalid taxonomy.

This is how I register my custom taxonomy

function my_custom_post_type_and_taxonomy() {

    // Register Custom Taxonomy - theme for posts
    $labels = array(
        'name'                       => _x( 'Themes', 'Taxonomy General Name', 'text_domain' ),
        'singular_name'              => _x( 'Theme', 'Taxonomy Singular Name', 'text_domain' ),
        'menu_name'                  => __( 'Theme', 'text_domain' ),
        'all_items'                  => __( 'All Themes', 'text_domain' ),
        'parent_item'                => __( 'Parent Theme', 'text_domain' ),
        'parent_item_colon'          => __( 'Parent Theme:', 'text_domain' ),
        'new_item_name'              => __( 'New Theme Name', 'text_domain' ),
        'add_new_item'               => __( 'Add New Theme', 'text_domain' ),
        'edit_item'                  => __( 'Edit Theme', 'text_domain' ),
        'update_item'                => __( 'Update Theme', 'text_domain' ),
        'view_item'                  => __( 'View Item', 'text_domain' ),
        'separate_items_with_commas' => __( 'Separate themes with commas', 'text_domain' ),
        'add_or_remove_items'        => __( 'Add or remove themes', 'text_domain' ),
        'choose_from_most_used'      => __( 'Choose from the most used themes', 'text_domain' ),
        'popular_items'              => __( 'Popular Items', 'text_domain' ),
        'search_items'               => __( 'Search themes', 'text_domain' ),
        'not_found'                  => __( 'Not Found', 'text_domain' ),
        'no_terms'                   => __( 'No items', 'text_domain' ),
        'items_list'                 => __( 'Items list', 'text_domain' ),
        'items_list_navigation'      => __( 'Items list navigation', 'text_domain' ),
    );
    $args = array(
        'labels'                     => $labels,
        'hierarchical'               => true,
        'public'                     => true,
        'show_ui'                    => true,
        'show_admin_column'          => true,
        'show_in_nav_menus'          => true,
        'show_tagcloud'              => true,
    );
    register_taxonomy( 'theme', array( 'post', 'podcast' ), $args );    
}
add_action( 'init', 'my_custom_post_type_and_taxonomy', 0 );

Answer 1

Using this REST API End Point https://example.com/wp-json/wp/v2/taxonomies I found I had 3 Taxonomies available post_tag, category and series.

After seeing series, a custom post type, I knew I had something missing in my code.

I finally found an article Working with Custom Post Types in WP-API v2 which describes my issue. Apparently in API V2 there are some new arguments added in for the REST API. The specific argument I needed was 'show_in_rest' => true, and by adding it into my arguments array resolved my issue.

$args = array(
    'labels'       => $labels,
    ...
    'show_in_rest' => true,
    ...
);
register_taxonomy( 'theme', array( 'post', 'resource', 'podcast' ), $args );

More information about this can be found in the codex https://developer.wordpress.org/reference/functions/register_taxonomy/