gerred

class Delivery < ActiveRecord::Base
  belongs_to :deliverable, :polymorphic => true
  belongs_to :receivable, :polymorphic => true
  has_many :delivery_impressions
  has_many :users, :through => :delivery_impressions

  belongs_to :sender, :class_name => 'User', :foreign_key => :sender_id
end

class DeliveryImpression < ActiveRecord::Base