A different way to write a service object. Made in response to http://re-factor.com/blog/2013/09/27/slow-tests-are-the-symptom-not-the-cause/

AddsUserToList.rb

class AddsUserToList
  attr_reader :notifies_user_klass, :user_klass
  def initialize(user_klass = User, notifies_user_klass = NotifiesUser)
    @user_klass = user_klass
    @notifies_user_klass = notifies_user_klass
  end
 
  def add(params)
    user_klass.find_by_username!(params.fetch(:username).tap do |user|
      notifies_user_klass.(user, params.fetch(:mailing_list_name))
      user.add_to_mailing_list(params.fetch(:mailing_list_name))
    end
  end
  
end

MailingListsController.rb

class MailingListsController < ApplicationController
  respond_to :json
  def add_user
    user = AddsUserToList.new.add(username: params[:username], mailing_list_name: 'blog_list')
    respond_with user
  end
end