issue.rb

class Issue < ActiveRecord::Base
  has_many :line_items, :as => :itemable
  has_many :orders, :through => :line_items
end

line_item.rb

class LineItem < ActiveRecord::Base
  belongs_to :order
  belongs_to :itemable, :polymorphic => true
end

offer.rb

class Offer < ActiveRecord::Base
  has_many :line_items, :as => :itemable
  has_many :orders, :through => :line_items
end

order.rb

class Order < ActiveRecord::Base
  belongs_to :user

  has_many :line_items
  has_many :issues, :through => :line_items, :source => :itemable, :source_type => 'Issue'
  has_many :offers, :through => :line_items, :source => :itemable, :source_type => 'Offer'
end

user.rb

class User < ActiveRecord::Base
  belongs_to :user_session
  has_many :orders, :conditions => "status = 'Completed'"

## Source de mes soucis :
  has_many :issues, :through => :orders, :source => :issue, :conditions => "status = 'Completed'"
  has_many :offers, :through => :orders, :source => :offer, :conditions => "status = 'Completed'"
end