app_share.rb

class AppShare < ActiveRecord::Base
  belongs_to :app
  belongs_to :sharer, class_name: 'User'
  belongs_to :sharee, class_name: 'User'

  has_many :notifications, foreign_key: :notified_object_id, conditions: {notified_object_type: 'AppShare'}, dependent: :destroy

  after_commit :create_notification, on: :create

  def create_notification
    subject = "#{sharer.name} has given you access to #{app.name}"
    body = "#{sharer.name} (#{sharer.email}) has granted you access to the #{app.name} account '#{app.username}'."
    sharee.notify(subject, body, self)
  end
end

mailboxer.rb

Mailboxer.setup do |config|
  config.uses_emails = false
  config.search_enabled = false
end

resque_schedule.yml

ScheduleSendNotificationsJob:
  every: 1m

schedule_send_notifications_job.rb

class ScheduleSendNotificationsJob < Job
  @queue = :high

  def perform
    # Find users with outstanding notifications, and deal with each in
    # a different job
    user_ids = User.
      joins(:receipts).
      where('confirmed_at IS NOT NULL').
      where(receipts: {is_read: false}).
      select('DISTINCT users.id').
      map(&:id)

    user_ids.each do |user_id|
      SendNotificationsJob.create(id: user_id)
      Rails.logger.info "Scheduled a job to send notifications to user #{user_id}"
    end
  end
end

send_notifications_job.rb

class SendNotificationsJob < Job
  @queue = :high

  COOLDOWN_PERIOD = 4.minutes

  def perform
    user = User.find(options['id'])

    if !user.confirmed?
      Rails.logger.info "Not sending notifications to unconfirmed email #{user.email}"
      return
    end

    ActiveRecord::Base.transaction do
      receipts = user.receipts.where(is_read: false).lock(true)

      most_recent = receipts.map(&:created_at).max

      if most_recent && (most_recent < (Time.now - COOLDOWN_PERIOD))
        notifications = Notification.find(receipts.map(&:notification_id))
        Rails.logger.info "Sending #{notifications.size} notification(s) to #{user.email}"
        NotificationsMailer.notifications_email(user, notifications).deliver
        receipts.update_all(is_read: true)
      else
        Rails.logger.info "Waiting before sending notifications to #{user.email}"
      end
    end
  end
end

user.rb

class User < ActiveRecord::Base
  acts_as_messageable
end