update field inside custom “set_object_terms” hook function

Bonjour!

I am trying to rewrite this custom set_object_terms hook function.
What I am trying to achieve in addition to Automatically check child categories/terms when parent category is checked as this function already is doing, is to also update the GeoDirectory field post_category with the same data as the updated values for: $categories = get_the_terms( $post->ID, 'gd_placecategory' );

Note that Geo Directory has it own set of get/delete/save functions, I have tested them from a template . What I have tried in order to solve this issue is to create an $merge and $diff variable for add and remove, and then trying to add this to the post_category field by using the geodir_save_post_meta() function. I have also tried to perform the update by using the $wpdb->update() as well.

add_action( 'set_object_terms', 'so_60079535_toggle_child_terms', 10, 6 );
function so_60079535_toggle_child_terms( $object_id, $terms, $tt_ids, $taxonomy, $append, $old_tt_ids ){

    global $wpdb;

    // Abort if this is an autosave/backup
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;

    // Abort if no ids are set from before or now
    if( empty($tt_ids) && empty($old_tt_ids) )
        return;

    // Only do things if this post is published (front facing)
    $post_status = get_post_status( $object_id );
    if( $post_status != 'publish' )
        return;

    // What terms where ADDED, and which were REMOVED?
    $added_terms   = array_diff_once( $tt_ids, $old_tt_ids );
    $removed_terms = array_diff_once( $old_tt_ids, $tt_ids );

        // Any terms ADDED this time?
    if( !empty($added_terms) ){
        $add = array();

        foreach( $added_terms as $added_term ){

            // Do any of these added terms have children?
            if( $added_child_terms = get_term_children( $added_term, $taxonomy ) ){
                // Append those children
                wp_set_object_terms( $object_id, $added_child_terms, $taxonomy, true );
                $add[] = $added_child_terms;
            }
        }

        if( !empty($add) ){

            $add = $add[0];
            $merge = array_merge($old_tt_ids,$add);  
            $merge = implode(",",$merge);

            if( !empty($merge) ){

                geodir_save_post_meta($object_id, 'post_category', $merge);
                //$wpdb->update( 'wp_geodir_gd_subscription_detail', array( 'post_category' => $merge), array('post_id' => $object_id ));

            }
        }
    }

    // Any terms REMOVED?
    if( !empty($removed_terms) ){
         $remove = array();

        foreach( $removed_terms as $removed_terms ){

            // Do any of the removed terms have children?
            if( $removed_child_terms = get_term_children( $removed_terms, $taxonomy ) ){
                // Remove them all
                wp_remove_object_terms( $object_id, $removed_child_terms, $taxonomy, true );
                $remove[] = $removed_child_terms;
            }
        }

        if( !empty($remove) ){

            $remove = $remove[0];    
            $diff = array_diff($old_tt_ids,$remove);  
            $diff = implode(",",$diff);

            if( !empty($diff) ){

                geodir_save_post_meta($object_id, 'post_category', $diff );
                //$wpdb->update( 'wp_geodir_gd_subscription_detail', array( 'post_category' => $diff), array('post_id' => $object_id ));

            }
        }

    }
}

When either the geodir_save_post_meta() function or the $wpdb->update() is added to the function, this custom function does run, but only the terms are changed. None of the set_object_terms generated terms is added to the post_category field.

It seems to me that the set_object_terms hook is not the place to run a update from. Still I need this hook to perform the Automatically check child categories/terms when parent category is checked.

How can I both Automatically check child categories/terms when parent category is checked and update the GeoDirectory field post_category at the same time?