rails--rails/activemodel/lib/active_model/conversion.rb

module ActiveModel
  # == Active Model Conversions
  #
  # Handles default conversions: to_model, to_key and to_param.
  #
  # == Example
  #
  # Let's take for example this non persisted object.
  #
  #   class ContactMessage
  #     include ActiveModel::Conversion
  #
  #     # ContactMessage are never persisted in the DB
  #     def persisted?
  #       false
  #     end
  #   end
  #
  #   cm = ContactMessage.new
  #   cm.to_model == self #=> true
  #   cm.to_key           #=> nil
  #   cm.to_param         #=> nil
  #
  module Conversion
    # If your object is already designed to implement all of the Active Model 
    # you can use the default to_model implementation, which simply returns 
    # self.
    # 
    # If your model does not act like an Active Model object, then you should 
    # define <tt>:to_model</tt> yourself returning a proxy object that wraps 
    # your object with Active Model compliant methods.
    def to_model
      self
    end

    # Returns an Enumerable of all (primary) key attributes or nil if 
    # persisted? is false
    def to_key
      persisted? ? [id] : nil
    end

    # Returns a string representing the object's key suitable for use in URLs,
    # or nil if persisted? is false
    def to_param
      to_key ? to_key.join('-') : nil
    end
  end
end