Your class, no refactoring

photo.rb

class Photo < ActiveRecord::Base
  belongs_to :user
  has_many :notifications
  
  has_attached :picture
  
  after_create :notify_users

  def notified_users
    User.where("id in (?)", self.notifications.map(&:user_id))
  end
  
  private
  def notify_users
    UserNotifier.new(self).notify
  end
end