notification.rb

class Notification < ActiveRecord::Base
  belongs_to :sender, class_name: "User"
  belongs_to :receiver, class_name: "User"
  
  validates :sender_id, presence: true
  validates :receiver_id, presence: true
  
  attr_accessible :sender_id, :receiver_id
end

user.rb

class User < ActiveRecord::Base  
  has_many :notifications, foreign_key: "sender_id", dependent: :destroy
  has_many :reverse_notifications, foreign_key: "receiver_id", class_name: "Notification", dependent: :destroy