And after some simple refactoring

new_code.rb

module Notifiable
  def self.included(klass)
    klass.has_many :notifications
  end
  
  def notified_users
    User.where("id in (?)", self.notifications.map(&:user_id)
  end
end

class Post < ActiveRecord::Base
  include Notifiable
  belongs_to :user
end

class Photo < Post
  has_attached :picture
end

class NotifiableCreator
  def initialize(notifiable)
    @notifiable = notifiable
  end
  
  def create
    result = @notifiable.create
    if result
      UserNotifier.new(self).notify
    end
    result
  end
end