Threaded emails: Final result

notification_mailer.rb

class NotificationMailer < ApplicationMailer
  def review_notification(recipient, review)
    @review = review
    @recipient = recipient
    mail_settings = {
      to: "#{recipient.name} <#{recipient.email}>",
      subject: "Review: #{review.book.title}"
    }.merge(headers(review))
    mail(mail_settings)
  end

  private

  def headers(review)
    {
      in_reply_to: thread_id(review),
      references: thread_id(review)
    }.reject { |_, v| v.blank? }
  end
  
  def message_id(review)
    return "" if review.nil?

    "<notification-#{review.id}-#{review.created_at.to_i}@#{ENV["domain"]}>"
  end
  
  def reviews_in_thread(review)
    Review.where.not(id: review.id).where("book_id", review.book_id).order(:created_at)
  end

  def thread_id(review)
   first_review = reviews_in_thread(review).first
   message_id(first_review)
  end
end