yii中数据库迁移出错

I've been trying to do a Database Migration, but I keep getting an error message:

Error : setting unknown property: yii\db\Connection::mailer

in my main-local I have the code:

 'mailer' => [
            'class' => 'yii\swiftmailer\Mailer',
            'viewPath' => '@common/mail',
        ],

What should I do?