Customize delayed_job for sending email, create association between the job and the object, so that can quickly find the object's jobs.

1_add_owner_to_delayed_jobs.rb

# db/migrate/add_owner_to_delayed_jobs.rb
class AddOwnerToDelayedJobs < ActiveRecord::Migration
  def self.up
    add_column :delayed_jobs, :owner_type, :string
    add_column :delayed_jobs, :owner_id, :integer

    add_index :delayed_jobs, [:owner_type, :owner_id]
  end

  def self.down
    remove_index :delayed_jobs, [:owner_type, :owner_id]

    remove_column :delayed_jobs, :owner_type
    remove_column :delayed_jobs, :owner_id
  end
end

2_migrate_delayed_job_data_to_add_owner.rb

# db/migrate/migrate_delayed_job_data_to_add_owner.rb
class MigrateDelayedJobDataToAddOwner < ActiveRecord::Migration
  def self.up
    Delayed::Job.where(:owner_id => nil).each do |job|
      begin
        obj = YAML::load(job.handler).args.first
        class_name = obj.class.name
        obj_id = obj.id
        # assign owner
        job.owner_type = class_name
        job.owner_id = obj_id
        job.save
      rescue
        job.delete
      end
    end
  end

  def self.down
  end
end

3_delayed_job.rb

# config/initializers/delayed_job.rb
class Delayed::Job < ActiveRecord::Base
  belongs_to :owner, :polymorphic => true
  attr_accessible :owner, :owner_type, :owner_id
end

4_active_record.rb

# config/initializers/active_record.rb
class ActiveRecord::Base
  has_many :jobs, :class_name => "::Delayed::Job", :as => :owner

  def send_mail_of(method, time=nil, queue=nil, *args)
    Delayed::Job.transaction do
      job = Delayed::Job.enqueue(Delayed::PerformableMailer.new(Notifier, 
             method.to_sym, args), :priority => 0, :run_at => time, :queue => queue)
      job.owner = self
      job.save
    end
  end

  def self.jobs
    # to address the STI scenario we use base_class.name. 
    # downside: you have to write extra code to filter STI class specific instance.
    Delayed::Job.find_all_by_owner_type(self.base_class.name)
  end
end

5_user_auto_emails.rb

# Usage, the :auto_special_offer_email is a email function defined in Notifier.rb
class User < ActiveRecord::Base
  def send_auto_emails
    send_mail_of(:auto_special_offer_email, 15.days.from_now, "pre_register", self)
  end
end

# list all jobs of User
User.jobs

# list jobs of first user
User.first.jobs