protected function _extraColumns(App\Modules\Abstracts\Models\Blueprint $table)
 {
     $table->unsignedMediumInteger('user_id');
     $table->char('username', 50);
     $table->char('password', 100);
     $table->enum('type', Credential::getCredentialTypes());
     $table->index(['user_id'], 'user_id');
     $table->index(['username'], 'username');
     $table->index(['password'], 'password');
     $table->index(['type'], 'type');
 }