socialforge/app/models/course_message.rb

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

  # 多态 虚拟关联
  belongs_to :course_message ,:polymorphic => true
  belongs_to :course
  belongs_to :user
  has_many :message_alls, :class_name => 'MessageAll',:as =>:message,  :dependent => :destroy

  validates :user_id,presence: true
  validates :course_id,presence: true
  validates :course_message_id,presence: true
  validates :course_message_type, presence: true
  validates_length_of :content, :maximum => 100
  after_create :add_user_message

  def add_user_message
    self.message_alls << MessageAll.new(:user_id => self.user_id)
  end
end