So I needed some metadata to be in the rest API of a custom post_type.
The following adds the subtitle to the book post_type
<?php add_action('rest_api_init', function () { register_rest_field('book', 'subtitle', [ 'get_callback' => 'get_meta_data_for_rest', 'update_callback' => null, 'schema' => null, ]); }); function get_meta_data_for_rest($post, $field_name, $request) { // it's not always an object if (isset($post->id) && ! empty($post->id)) { $post_id = $post->id; } elseif (isset($post['id']) && ! empty($post['id'])) { $post_id = $post['id']; } else { return null; } return get_post_meta($post_id, $field_name, true); }