how can I set attribute of join model in a has many through association?

I have:

class Ruser < ActiveRecord::Base
  has_many :deliveries
  has_many :bags, :through => :deliveries

class Bag < ActiveRecord::Base
  has_many :deliveries
  has_many :rusers, :through => :deliveries

class Delivery < ActiveRecord::Base
  belongs_to :ruser
  belongs_to :bag

Delivery has an extra attribute :delivered_at of type date.

I have Ruser and Bag already populated.
I want to assign bags to rusers simultaneously setting delivered_at.
Doing user.bags << bags doesn't allow to set delivered_at.