public function actionCategory($id = null, $slug = null)
    {
        if (!is_numeric($id) || $id < 1 || empty($slug)) {
            $this->error(Yii::t('podium/flash', 'Sorry! We can not find the category you are looking for.'));
            return $this->redirect(['forum/index']);
        }
        $conditions = ['id' => $id, 'slug' => $slug];
        if ($this->module->user->isGuest) {
            $conditions['visible'] = 1;
        }
        $model = Category::find()->where($conditions)->limit(1)->one();
        if (empty($model)) {
            $this->error(Yii::t('podium/flash', 'Sorry! We can not find the category you are looking for.'));
            return $this->redirect(['forum/index']);
        }
        $this->setMetaTags($model->keywords, $model->description);
        return $this->render('category', ['model' => $model]);
    }