This was originally published on the Coherence blog on December 17th, 2013. It has been migrated here for historical purposes.

In case you haven’t been paying attention, Rails 4.1 beta was released today! While this is a minor release, there are still plenty of handy new features that’s worth getting excited about. Here are a few of my favourites, along with some thoughts on why I think they are useful.

(This post is based on my lightning talk at #VANRUBY, the original slides can be found here.)

Action Mailer Previews

Testing email templates in Rails has always been pretty painful. My current workflow involves:

  1. Make changes to the email template
  2. Deliver the email via the rails console
  3. Check the output in the browser
  4. Rinse and repeat

The Letter Opener gem makes this easier, but this is still far from ideal. Fortunately for us, @pixeltrix did the hard work to integrate the MailView gem into Rails 4.1. You can now easily create previews for your mailers and view them in the browser from http://localhost:3000/rails/mailers:

# In /test/mailers/previews/notifier_preview.rb
class NotifierPreview < ActionMailer::Preview
  def welcome
    # Mock up some data for the preview
    user =

    # Return a Mail::Message here (but don't deliver it!)

It should be noted that although the preview files live under the test directory by default (which can be changed via config.action_mailer.preview_path), it is actually run inside the development environment. So if you need to use gems like FactoryGirl to generate fake data, you’ll need to make sure they are added to the development group in your Gemfile as well.

If your app doesn’t currently have a test folder (i.e. rspec users), it might be tempting to change the default config.action_mailer.preview_path to something like /app/mailer/previews. However, please be aware that the /app folder is eager-loaded in production, so that’s probably not the best place to put these preview files.

Read more about this feature:

Active Record Enums

Have you ever used multiple boolean columns to compose a single complex state on your models? I’ve definitely done this before and things get out of hand really quickly.

Enums to the rescue!

class Bug < ActiveRecord::Base
  # Relevant schema change looks like this:
  # create_table :bugs do |t|
  #   t.column :status, :integer, default: 0 # defaults to the first value (i.e. :new)
  # end

  enum status: [ :new, :assigned, :in_progress, :resolved, :rejected, :reopened ]

  belongs_to :assignee, class_name: 'Developer'

  def assignee=(developer)
    if developer &&
      self.status = :assigned
      self.status = :new


Bug.resolved           # => a scope to find all resolved bugs

bug.resolved?          # => check if bug has the status :resolved

bug.resolved!          # => update! the bug with status set to :resolved

bug.status             # => a symbol describing the bug's status

bug.status = :resolved # => set the bug's status to :resolved

Internally, these states are mapped to integers in the database to save space. It’s also worth mentioning that the methods added by the enum macro are mixed-in via a module. This means you can easily override them in your model and use super to reach the original implementation.

There are a few caveats you should keep in mind when using this feature:

I. Despite its name, this feature doesn’t actually use the ENUM type that is implemented in certain databases. The mapping between states and their corresponding integers are maintained in the Ruby model file. This means that you should not change the order of the enum symbols once they are added. To remove unused states, you can use an explicit mapping:

class Bug < ActiveRecord::Base
  enum status: {
    new: 0,
    in_progress: 2,
    resolved: 3,
    rejected: 4,
    reopened: 5

II. Avoid using the same names inside different enums in the same class! Doing so will leave Active Record very confused!

class Bug < ActiveRecord::Base
  enum status: [ :new, ... ]
  enum code_review_status: [ :new, ... ] # WARNING: Don't do this!

III. If you need to write custom scopes to query the enum columns, you would have to pass the integers instead of the symbols. You can access the enum-integer mapping via a constant added by the macro:

class Bug < ActiveRecord::Base
  scope :open, -> {
    where('status <> ? OR status <> ?', STATUS[:resolved], STATUS[:rejected])

IV. Currently, the dirty tracking methods (e.g. status_was?) have not been updated to work with enums yet (they currently return the mapped integer instead of the symbols). This should be fixed before the final release. (See #13267 for the progress.)

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Action Pack Variants

As web developers, we are well aware that we have fully transitioned into the post-PC era. As much as I love responsive design, it is not a silver bullet for the multi-device web. In many cases, it is more appropiate to tailor your views to serve the most relevant content and workflow for specific device categories.

With Action Pack Variants, this will become much easier in Rails 4.1:

class ApplicationController < ActionController::Base
  before_action :detect_device_variant


    def detect_device_variant
      case request.user_agent
      when /iPad/i
        request.variant = :tablet
      when /iPhone/i
        request.variant = :phone

class PostController < ApplicationController
  def show
    @post = Post.find(params[:id])

    respond_to do |format|
      format.html               # /app/views/posts/show.html.erb         # /app/views/posts/show.html+phone.erb
      format.html.tablet do
        @show_edit_link = false

This example sets up a before_action filter to match the User-Agent HTTP header against certain keywords, and assign the request.variant accordingly. By specifying the supported variants in the respond_to block, Rails will render the appropiate template for the specific format and variant combination. It also allows you to run additional variant-specific code by passing a block.

In fact, you can even skip the declaration - if you have the appropiate template in your views directory, Rails will automatically pick it up. On the other hand, if a variant does not have a specialized template, Rails will fallback to the default template for the format (i.e. show.html.erb). This allows you to share a template between two variants. In this example, the tablet variant will reuse the default template if /app/views/posts/show.html+phone.erb is absent.

Although most examples use the User-Agent header to showcase this feature, it’s worth noting that the actual implementation in Rails is completely agnostic from that. request.variant can be assigned any time before the template is rendered based on arbitrary conditions, such as the request (sub)domain, HTTP headers, session data, or even the result of a coin flip.

This makes the feature very flexible, and can potentially be used for many things such as API versioning, A/B testing, or even feature rollouts!

Read more about this feature:

Application Message Verifier

Rails 4.1 also included a built-in helper to generate signed messages with HMAC. The message verifier was previously used to power things like signed cookies, but it is now much easier to use it for other purposes.

For example, you can implement a stateless “reset password” feature without having to store any tokens in the database:

class User < ActiveRecord::Base
  class << self
    def verifier_for(purpose)
      @verifiers ||= {}
      @verifiers.fetch(purpose) do |p|
        @verifiers[p] = Rails.application.message_verifier("#{}-#{p.to_s}")

  def reset_password_token
    verifier = self.class.verifier_for('reset-password') # Unique for each type of messages

  def reset_password!(token, new_password, new_password_confirmation)
    # This raises an exception if the message is modified
    user_id, timestamp = self.class.verifier_for('reset-password').verify(token)

    if timestamp >
      self.password = new_password
      self.password_confirmation = new_password_confirmation
      # Token expired
      # ...

class Notifier < ActionMailer::Base
  def reset_password(user)
    @user = user
    @reset_password_url = password_reset_url(token: @user.reset_password_token)
    mail(to:, subject: "Your have requested to reset your password")

That way, everything that is required to fufill the password reset request is included in the link, nothing need to be stored in the database. This can also be used for things like OAuth (the state parameter).

When using this feature, it is important to conisder possible replay attacks. In the example above, if we did not include a timestamp to check for expiration, the same URL can be used to reset the user’s password at any time if the email ended up in the wrong hands!

Also, the key used to sign the message is derived from your application’s secret_key_base and the “salt” you passed ("User-reset-password" in the example). Changing either will invalidate any previously signed messages.


Depending on the gems you use, an average Rails app probably take around ~5 seconds to boot. That’s 5 seconds wasted every time you run your tests, even when you are just running a single isolated test case! If you are following TDD, you are probably doing this 50 times a day. That’s 5 days wasted in the last five years!

Luckily for all of us, new applications generated with Rails 4.1 comes with built-in integration with the Spring application preloader.

Spring works by keeping your application running in the background so you don’t need to boot it every time you run a test, rake task or migration. If you are familiar with Zeus gem or the Spork gem, this should sound famaliar. However, it transparently wraps common Rails commands (rake and rails by default) with binstubs, so if you have ./bin in your PATH, things should automagically become much faster for you, no actions required!

I tried this on the Caliper dashboard, and we can save almost 5 seconds between test runs when spring is loaded. Can I get my 5 extra days off now? ;)

You can read about how it works in the Spring README as well as how to install this for existing applications.

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Even More Features

This is just scratching the surface of this release. There are many other features that you might find useful, such as a secrets.yml for all your secrets, time travelling in tests, better JSON handling, Module#concerning, to_param macro and more. I encourage you to checkout the release notes for a full list of changes!

#VANRUBY Upgrade Clinic

We will be running our 4.1 upgrade clinic again at the #VANRUBY hack night today. Stop by and say hi if you are in town!

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