public function importFields($model, $fields = null)
    {
        $this->model = $model;

        if ($fields === false) {
            return;
        }

        if (!$fields) {
            if ($model->only_fields) {
                $fields = $model->only_fields;
            } else {
                $fields = [];
                // get all field-elements
                foreach ($model->elements as $field => $f_object) {
                    if ($f_object instanceof \atk4\data\Field) {
                        $fields[] = $f_object;
                    }
                }
            }
        }

        if (!is_array($fields)) {
            $fields = [$fields];
        }

        // import fields one by one
        foreach ($fields as $field) {
            $this->importField($field);
        }

        return $this;
    }