Automatically set/assign child taxonomy terms for hierarchical taxonomies in WordPress (job listings, with job_listing_category taxonomy)

functions.php

<?php
add_action( 'set_object_terms', 'auto_set_child_terms', 9999, 6 );
/**
 * Automatically set/assign child taxonomy terms to posts
 *
 * This function will automatically set child taxonomy terms whenever a parent term is set on a post,
 * with the option to configure specific post types, and/or taxonomies.
 *
 *
 * @param int    $object_id  Object ID.
 * @param array  $terms      An array of object terms.
 * @param array  $tt_ids     An array of term taxonomy IDs.
 * @param string $taxonomy   Taxonomy slug.
 * @param bool   $append     Whether to append new terms to the old terms.
 * @param array  $old_tt_ids Old array of term taxonomy IDs.
 */
function auto_set_child_terms( $object_id, $terms, $tt_ids, $taxonomy, $append, $old_tt_ids ) {

	/**
	 * We only want to move forward if there are taxonomies to set
	 */
	if ( empty( $tt_ids ) ) {
		return false;
	}
	/**
	 * Set specific post types to only set parents on.  Set $post_types = FALSE to set parents for ALL post types.
	 */
	$post_types = array( 'job_listing' );
	if ( $post_types !== false && ! in_array( get_post_type( $object_id ), $post_types ) ) {
		return false;
	}

	/**
	 * Set specific post types to only set parents on.  Set $post_types = FALSE to set parents for ALL post types.
	 */
	$tax_types = array( 'job_listing_category' );
	if ( $tax_types !== false && ! in_array( $taxonomy, $tax_types ) ) {
		return false;
	}

	foreach ( $tt_ids as $tt_id ) {
		$children = get_term_children( $tt_id, $taxonomy );
		if ( ! empty( $children ) ) {
			wp_set_post_terms( $object_id, $children, $taxonomy, true );
		}
	}
}

Hey Myles - wondering if this gist could work in the reverse? I can't think of a particular use case for your way around but in my case I want Parent taxonomies to be automatically set when any of its children are selected, for example; if someone selects "London" or "Manchester" then the parent "United Kingdom" would automatically be set?

Hey Myles - wondering if this gist could work in the reverse? I can't think of a particular use case for your way around but in my case I want Parent taxonomies to be automatically set when any of its children are selected, for example; if someone selects "London" or "Manchester" then the parent "United Kingdom" would automatically be set?

@JasonBroderick Yup right here:
https://gist.github.com/tripflex/65dbffc4342cf7077e49d641462b46ad