Do not pass records to your Rails mailers

The problem

Sutori has a system of inviting users to your story. As the author, you give the email of the invitee. In the backend, it then creates a record of type Invitation and sends the invitation email. The mailer looks like this:

class InvitationMailer < ApplicationMailer
  def invitation(invitation:)
    mail(
      to: invitation.email,
      subject: "You've been invited!"
    )
  end
end

Then we started getting the following errors:

ActiveJob::DeserializationError: Error while trying to deserialize arguments:
Couldn't find Invitation with 'id'=12345

The crucial part is that the email is sent asynchronously through ActiveJob. This is the sequence of events that caused this error:

1. The user creates the invitation.
2. In the backend, the Invitation record is created and the email is added to the queue of background jobs.
3. The user deletes the invitation, maybe because he made a typo in the email addres.
4. The Invitation record is deleted from the database.
5. It was a busy period for the background jobs so the mail from step 2 is only now sent. Then when Rails tries to load the Invitation record from the database, it fails with the error above.

The consequences are not severe in the sense that the mail simply won’t get sent because of the exception. However, it’s never a good thing to have preventable exceptions in your log as it clutters it and removes the attention from real exceptions. Therefore, it should be cleaned up. This is further aggravated by the fact that some background processors, e.g. Sidekiq, retry a job multiple times before it’s killed. So this exception will show up multiple times for the same email.

The solution

The solution is simple: instead of passing the full record to the mailer, we only pass it an ID. We then abort if the record does not exist in the database. So the mailer interface becomes:

class InvitationMailer < ApplicationMailer
  def invitation(invitation_id:)
    invitation = Invitation.find_by(id: invitation_id)
    return if invitation.nil?

    mail(
      to: invitation.email,
      subject: "You've been invited!"
    )
  end
end

Thanks to this PR that was merged into Rails back in 2012, the mail won’t get rendered if no call to mail() is made.

Bottom line

The bottom line is that you can never assume the presence of records in background jobs. Since they are processed asynchronously, the state of the database will likely have changed by the time the job is processed.