public function parentRelationActiveByDefault(string $relation): ModelList
    {
        $this->main['default_top_relation'] = $relation;

        if ( ! array_get($this->main, 'parents')) {
            // Assume the relation (= the field) if no parents set yet.

            $this->main['parents'] = [
                [
                    'relation' => $relation,
                    'field'    => $relation,
                ],
            ];
        }

        return $this;
    }