WordPress Rest API: Adding custom fields to custom post types

wpjm_add_custom_field.php

<?php

/**
	 * Register a custom field (using a bit of Mixtape)
	 *
	 * @throws WPJM_REST_Exception When bad things happen.
	 */
	function register_job_listing_custom_fields() {
		$env = $this->wpjm_rest_api->environment();
		$string_type = $env->type()->definition( 'string' );
		$description = __( 'Application email or Url', 'wp-job-manager' );

		$builder = new WPJM_REST_Model_Field_Declaration_Builder();

		$field_declaration = $builder->named( '_application' )
			->dto_name( 'application_email_or_url' )
			->description( $description )
			->derived( 'wpjm_get_application_meta' )
			->typed( $string_type )
			->build();

		register_rest_field( 'job_listing', $field_declaration->get_data_transfer_name(), array(
			'get_callback'    => $field_declaration->get_map_from(),
			'update_callback' => null, // TODO: updateable derived field declarations?
			'schema'          => array( $field_declaration, 'as_item_schema_property' ), // Puke: no Callables allowed here! Will file a bug with wp-core.
		) );
	}

add_action( 'rest_api_init', 'register_job_listing_custom_fields' )

	/**
	 * @return string
	 */
	function wpjm_get_application_meta() {
		global $post;
		return get_post_meta( $post->ID, '_application', true );
	}

wpjm_rest_add_job_listings.php

<?php

/**
 * Add REST API support to job listings
 */
function register_job_listings_for_rest() {
  global $wp_post_types;
  $post_type_name = 'job_listing';
  if( ! isset( $wp_post_types[ $post_type_name ] ) ) {
    return;
  }
  $job_listings = $wp_post_types[ $post_type_name ];
    $job_listings->show_in_rest = true;
    $job_listings->rest_base = 'job-listings';
}

add_action( 'init', array( 'register_job_listings_for_rest' ), 25 );