WP-API/WP-API

<?php

class WP_REST_Post_Meta_Fields extends WP_REST_Meta_Fields {
    /**
     * Post type to register fields for.
     *
     * @var string
     */
    protected $post_type;

    /**
     * Constructor.
     *
     * @param string $post_type Post type to register fields for.
     */
    public function __construct( $post_type ) {
        $this->post_type = $post_type;
    }

    /**
     * Get the object type for meta.
     *
     * @return string
     */
    protected function get_meta_type() {
        return 'post';
    }

    /**
     * Get the type for `register_rest_field`.
     *
     * @return string Custom post type slug.
     */
    public function get_rest_field_type() {
        return $this->post_type;
    }
}