trustieforge/app/models/course_message.rb

class CourseMessage < ActiveRecord::Base
  attr_accessible :course_id, :course_message_id, :course_message_type, :user_id, :viewed

  # 多态 虚拟关联
  belongs_to :course_message ,:polymorphic => true
  belongs_to :course
  belongs_to :user
  validates :user_id,presence: true
  validates :course_id,presence: true
  validates :course_message_id,presence: true
  validates :course_message_type, presence: true
end