Exporting bidirectional relationship fields

The below PHP, when installed as a plugin, will export post type “page” with IDs for relationships stored in field “film_actors”:

<?php
/**
 * Plugin Name: Pods — Relationship Export
 * Description: Export a post type to CSV with <a href="admin-ajax.php?action=export-relationship">this link</a>.
 * Plugin Author: ??δ???
 * Author URI: https://pd.cm/
 * Version: 1
 */

add_action(
	'wp_ajax_export-relationship',
	function(){
		global $post;

		$post_type = 'page';
		$relationship_field_key = 'film_actors';

		header( 'Content-Type: text/csv' );
		header( 'Content-Disposition: attachment; filename=' . $post_type . '.csv' ); 

		$query = new WP_Query(
			[
				'post_type' => $post_type,
				'posts_per_page' => -1,
				'post_status' => 'publish',
			]
		);

		ob_start();
		$output = fopen('php://output', 'w');

		$headers = [
			'ID',
			'post_title',
			'post_content',
			$relationship_field_key,
		];

		fputcsv( $output, $headers );

		while ( $query->have_posts() ) {
			$query->the_post();

			fputcsv(
				$output,
				[
					$post->ID,
					$post->post_title,
					$post->post_content,
					implode( ',', (array) get_post_meta( $post->ID, $relationship_field_key, false ) )
				]
			);
		}

		fclose( $output );

		echo ob_get_clean();

		exit;
	}
);

Once uploaded as a plugin, a link to download the CSV displays in the plugin description.

Thank you Paul