public function getRelationLabel($activeRelation, $relation)
    {
        $modelClass = $activeRelation->modelClass;
        if (isset($this->cachedRelationLabels[$modelClass][$relation])) {
            return $this->cachedRelationLabels[$modelClass][$relation];
        }
        if (isset($this->relationLabels[$relation])) {
            $label = $this->relationLabels[$relation];
        } else {
            /** @var ActiveRecord $relationModel */
            $relationModel = new $modelClass;
            $label = $relationModel->getCrudLabel($activeRelation->multiple ? 'relation' : 'default');
        }
        return $this->cachedRelationLabels[$modelClass][$relation] = $label;
    }