public static function get_prerequisite( $post_id ) {
		$post_type = get_post_type( $post_id );

		if ( 'course' === $post_type ) {
			$course = new LLMS_Course( $post_id );
			return $course->get_prerequisite();

		} elseif ( 'lesson' === $post_type ) {
			$lesson = new LLMS_Lesson( $post_id );
			return $lesson->get_prerequisite();
		}
	}