Use Observer

Bad Smell

class Project < ActiveRecord::Base
  after_create :send_create_notifications

  private
    def send_create_notifications
      self.members.each do |member|
        ProjectMailer.deliver_notification(self, member)
      end
    end
end

In this example, we use the model callback to send email notifications after creating a project. It would be better to use Observer because the email notification is a subsystem whose functionality is used by other classes.

Refactor

class Project < ActiveRecord::Base
  # nothing here
end

class NotificationObserver < ActiveRecord::Observer
  observe Project

  def after_create(project)
    project.members.each do |member|
      ProjectMailer.deliver_notice(project, member)
    end
  end
end

Using the Observer, there is no necessary for Project to know about email notification, the NotificationObserver is responsible for sending emails after create a project, it may also sending emails for creating or updating an issue. It's much easier to maintain email notifications in one Observer than some email notification functions distributed over different models.

Updated: thanks @carlosantoniodasilva