gistfile1.txt

<p>
  <%= f.label :roles %><br />
  <% for ns in User::NOTIFICATION_SETTINGS %>
    <%= check_box_tag "user[notification_settings][]", ns, @user.notification_settings.include?(ns) %>
    <%=h notification_setting.humanize %><br />
  <% end %>
</p>

post_observer.rb

class PostObserver << ActiveRecord::Observers
  after_create(post)
    if post.user.notification_settings.include?(:post_created)
      send_the_mother_fucker
    end
  end
end

user.rb

class User

  NOTIFICATION_SETTINGS = [:post_created,
                           :post_closed, 
                           :reply_created, 
                           :reply_helpful, 
                           :offer_created, 
                           :offer_accepted, 
                           :offer_rejected, 
                           :offer_cancelled
                           :feedback_available,
                           :feedback_left, 
                           :order_created, 
                           :alert]

  def notification_settings=(settings)
    self.ns_mask = (settings & NOTIFICATION_SETTINGS).sum { |ns| 1 << NOTIFICATION_SETTINGS.index(ns) }
  end
  
  def notification_settings
    NOTIFICATION_SETTINGS.reject { |ns| (ns_mask || 0)[NOTIFICATION_SETTINGS.index(ns)].zero? }
  end

end