命名空间缺失

我在文件夹前端/迁移中有以下课程

use yii\db\Schema;

class m170727_180101_Bewerbungen extends \yii\db\Migration
{
    public function safeUp()
    {
        $tableOptions = null;
        if ($this->db->driverName === 'mysql') {
            $tableOptions = 'CHARACTER SET utf8 COLLATE utf8_general_ci ENGINE=InnoDB';
        }

        $this->createTable('bewerbungen', [
            'bew_id' => $this->primaryKey(),
            'datum' => $this->date()->notNull(),
            'firma' => $this->string(100)->notNull(),
            'rechtsart' => $this->integer(11),
            'stadt' => $this->string(100)->notNull(),
            'plz' => $this->integer(11)->notNull(),
            'strasse_nr' => $this->string(100),
            'ansprech_person' => $this->string(100),
            'email' => $this->string(50)->notNull(),
            'feedback' => $this->integer(11),
            'bemerkungen' => $this->string(150),
            'FOREIGN KEY ([[feedback]]) REFERENCES nachricht ([[id_message]]) ON DELETE CASCADE ON UPDATE CASCADE',
            'FOREIGN KEY ([[rechtsart]]) REFERENCES rechtsform ([[id_recht]]) ON DELETE CASCADE ON UPDATE CASCADE',
            ], $tableOptions);

    }

    public function safeDown()
    {
        $this->dropTable('bewerbungen');
    }
}

每次尝试读取方法safeUp()都会抛出错误:

在以下文件中找不到“frontend\migrations\m170727\u 180101\u Bewerbungen”:E:\xampp\htdocs\Yii2\u Mail/frontend/migrations/m170727\u 180101\u Bewerbungen.php。命名空间缺失**

namespace frontend\migrations; ...

$connect=new m170727_180101_Bewerbungen();
$connect->safeUp(); ...

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

这到底是什么? 同样的错误如下:

$connect=new \frontend\migrations\m170727_180101_Bewerbungen();