```ruby
class ActivityNotification::MailerPreview < ActionMailer::Preview

  def send_notification_email_single
    target_notification = ActivityNotification::Notification.where(group: nil).first
    ActivityNotification::Mailer.send_notification_email(target_notification)
  end

  def send_notification_email_with_group
    target_notification = ActivityNotification::Notification.where.not(group: nil).first
    ActivityNotification::Mailer.send_notification_email(target_notification)
  end

  def send_batch_notification_email
    target = User.find_by_name('Ichiro')
    target_notifications = target.notification_index_with_attributes(filtered_by_key: 'comment.default')
    ActivityNotification::Mailer.send_batch_notification_email(target, target_notifications)
  end

end```

Fix batch email preview in test rails app
user
```ruby
class ActivityNotification::MailerPreview < ActionMailer::Preview

  def send_notification_email_single
    target_notification = ActivityNotification::Notification.where(group: nil).first
    ActivityNotification::Mailer.send_notification_email(target_notification)
  end

  def send_notification_email_with_group
    target_notification = ActivityNotification::Notification.where.not(group: nil).first
    ActivityNotification::Mailer.send_notification_email(target_notification)
  end

  def send_batch_notification_email
    target = User.find_by_name('Ichiro')
    target_notifications = target.notification_index_with_attributes(filtered_by_key: 'comment.default')
    ActivityNotification::Mailer.send_batch_notification_email(target, target_notifications, 'batch.comment.default')
  end

end```
assistant