how to organize getting fat models.

invoice.rb

# app/models/invoice.rb
class Invoice < ActiveRecord::Base 
  has_many :items
end

# app/models/invoice.rb
class Item < ActiveRecord::Base 
  belongs_to :invoice
end

# app/models/invoice/item.rb
###################################
## refactor Item as follow
###################################
class Invoice::Item < ActiveRecord::Base 
  belongs_to :invoice
end