public function behaviors()
 {
     $sql = (new Query())->select('MAX(update_time)')->from(Post::tableName())->createCommand()->rawSql;
     return [['class' => 'yii\\filters\\PageCache', 'only' => ['rss', 'atom'], 'duration' => 24 * 60 * 60, 'dependency' => ['class' => DbDependency::className(), 'sql' => $sql]]];
 }
 public function down()
 {
     $this->dropTable(Post::tableName());
 }