public function get_related_post_ids( $post_type = null ) {
		$post_ids = $this->post_ids;

		if ( null !== $post_type ) {
			$candidates = array_map( 'get_post', $post_ids );
			$filtered   = wp_list_filter( $candidates, array( 'post_type' => $post_type ) );
			$post_ids   = wp_list_pluck( $filtered, 'ID' );
		}

		return $post_ids;
	}