application.rb

# ...
config.active_record.observers = :comment_observer
# ...

comment_observer.rb

class CommentObserver < ActiveRecord::Observer
  def after_create(comment)
    Notification.new_comment(comment).deliver
  end
end

comment_observer_spec.rb

require 'spec_helper'

describe CommentObserver do
  it "sends a notification mail after a new comment is created" do
    @comment = Factory.build(:comment)
    Notification.should_receive(:new_comment).with(@comment)
    @comment.save
  end
end

notification.rb

class Notification < ActionMailer::Base
  def new_comment(comment)
    @comment = comment
    mail(:to => ENV["ADMIN_EMAIL"], :from => ENV["ADMIN_EMAIL"], :subject => "New comment from #{@comment.name} on #{@comment.post.title}")
  end
end

notification_spec.rb

require "spec_helper"

describe Notification do
  describe "#new_comment" do
    before(:each) do
      ENV["ADMIN_EMAIL"] = 'admin@example.com'
      @comment = Factory(:comment)
      @email = Notification.new_comment(@comment).deliver
    end

    it "queues a message for new comments" do
      ActionMailer::Base.deliveries.should_not be_empty
    end

    it "sends email to and from the admin" do
      @email.to.should include(ENV["ADMIN_EMAIL"])
      @email.from.should include(ENV["ADMIN_EMAIL"])
    end
  end
end

rspec_output.txt

Notification
  #new_comment
    queues a message for new comments
    sends email to and from the admin

CommentObserver
  sends a notification mail after a new comment is created (FAILED - 1)




Failures:

  1) CommentObserver sends a notification mail after a new comment is created
     Failure/Error: @comment.save
     NoMethodError:
       undefined method `deliver' for nil:NilClass
     # ./app/models/comment_observer.rb:3:in `after_create'
     # ./spec/models/comment_observer_spec.rb:7:in `block (2 levels) in <top (required)>'