WP_Post::__isset( string $key )

	public function __isset( $key ) {
		if ( 'ancestors' == $key )
			return true;

		if ( 'page_template' == $key )
			return true;

		if ( 'post_category' == $key )
		   return true;

		if ( 'tags_input' == $key )
		   return true;

		return metadata_exists( 'post', $this->ID, $key );
	}