Has many through example

example.rb

class ServiceElement < ActiveRecord::Base
  attr_accessible :body, :title, :category, :source, :number, :file

  has_many :parts
  has_many :presentations, through: :parts, order: "presentations.date"
end

class Part < ActiveRecord::Base
  attr_accessible :position, :presentation_id, :service_element_id

  belongs_to :presentation
  belongs_to :service_element
end

class Presentation < ActiveRecord::Base
  attr_accessible :date, :theme, :service_element_ids

  has_many :parts
  has_many :service_elements, through: :parts, order: "position"

  default_scope order("date DESC")
  default_scope include: :parts
end