public function getModelRelations($model, $extraFields)
    {
        $relations = parent::getModelRelations($model, $extraFields);
        if (($relationName = Yii::$app->request->getQueryParam('_pjax')) !== null
            && ($relationName = substr($relationName, 1, -4)) !== ''
            && isset($relations[$relationName])
        ) {
            $headers = Yii::$app->request->getHeaders();
            $selection = [
                'add' => self::explodeEscaped(self::KEYS_SEPARATOR, $headers->get('X-Selection-add')),
                'remove' => self::explodeEscaped(self::KEYS_SEPARATOR, $headers->get('X-Selection-remove')),
            ];
            $relations[$relationName] = $this->addRelationSelection(
                $relations[$relationName],
                $selection
            );
        }
        foreach ($relations as $relationName => &$relation) {
            $relation['buttons'] = $this->getRelationButtons($relationName, $relation, $model);
        }
        return $relations;
    }