ACF bidirectional relationship

gistfile1.txt

class ConfigACF
{
    public function __construct()
    {
        // Configure ACF plugin custom functionality
        add_filter('acf/update_value/name=field_name', [$this, 'bidirectionalAcfUpdate'], 10, 3);
    }

    public function bidirectionalAcfUpdate($value, $post_id, $field)
    {
        // vars
        $field_name = $field['name'];
        $field_key = $field['key'];
        $global_name = 'is_updating_' . $field_name;

        // bail early if this filter was triggered from the update_field() function called within the loop below
        // - this prevents an inifinte loop
        if( !empty($GLOBALS[ $global_name ]) ) return $value;

        // set global variable to avoid inifite loop
        // - could also remove_filter() then add_filter() again, but this is simpler
        $GLOBALS[ $global_name ] = 1;

        // loop over selected posts and add this $post_id
        if( is_array($value) ) {

            foreach( $value as $post_id2 ) {

                // load existing related posts
                $value2 = get_field($field_name, $post_id2, false);

                // allow for selected posts to not contain a value
                if( empty($value2) ) {
                    $value2 = array();
                }

                // bail early if the current $post_id is already found in selected post's $value2
                if( in_array($post_id, $value2) ) continue;

                // append the current $post_id to the selected post's 'related_posts' value
                $value2[] = $post_id;

                // update the selected post's value (use field's key for performance)
                update_field($field_key, $value2, $post_id2);
            }
        }

        // find posts which have been removed
        $old_value = get_field($field_name, $post_id, false);

        if( is_array($old_value) ) {
            foreach( $old_value as $post_id2 ) {

                // bail early if this value has not been removed
                if( is_array($value) && in_array($post_id2, $value) ) continue;

                // load existing related posts
                $value2 = get_field($field_name, $post_id2, false);

                // bail early if no value
                if( empty($value2) ) continue;

                // find the position of $post_id within $value2 so we can remove it
                $pos = array_search($post_id, $value2);

                // remove
                unset( $value2[ $pos] );

                // update the un-selected post's value (use field's key for performance)
                update_field($field_key, $value2, $post_id2);

            }
        }

        // reset global varibale to allow this filter to function as per normal
        $GLOBALS[ $global_name ] = 0;

        return $value;
    }