./databyte


Testing in isolation, example in RABL

SOLID, Dependency Inversion principle, testing doubles, presenters, decorators, composites, oh my!

Now that I have your attention with a bunch of fancy words, let's apply them to a real problem. I've closed a lot of recent issues for RABL on "limitations" of how RABL supports syntax for multiple variables and conditions. The problem basically boils down to ignoring the SOLID principles.

RABL

If you're not familiar with RABL, it's a template rendering engine similar to ERB but tuned for structured outputs like JSON and XML.

RABL allows you to have a main object that everything is built around. However, there are pull requests for passing locals and passing objects to allow conditionals within blocks. There's also a slew of issues that mostly deal with syntax problems for making RABL do things you shouldn't allow your views to do in the first place.

Here's an example:

# post_controller.rb
def show
  @post = Post.find(params[:id])
end

# show.rabl
object @post

attributes :title, :body

child :author do
  # root_object is a hack for accessing @post
  attribute :name => :author_name unless root_object.author == current_user
end

node :publication_date do |post|
  if post.status == 'published'
    post.published_date
  end
end

Presenters, Decorators and Composites

If you're not sure what the differences between the wrappers are, I highly suggest reading an article by Thoughtbot that covers Decorators compared to Strategies, Composites, and Presenters. I'm going to combine two of those to make a presentable decorator:

# post_presenter.rb
class PostPresenter < Draper::Base
  decorates :post
  attr_reader :current_user

  def initialize(post, current_user)
    super(post)
    @current_user = current_user
  end

  def author_name
    author.name unless author == @current_user
  end

  def publication_date
    publication_date if status == 'published'
  end
end

# post_controller.rb
def show
  @post = PostPresenter.new(Post.find(params[:id]), current_user)
end

# show.rabl
object @post

attributes :title, :body, :author_name, :publication_date

Oh my, look at how easy that view is now!

If the attributes are nil, RABL will simply skip over them. It's like your own free Null Object. (Explanation of Null Object and a couple others just in case.)

If you don't want to use Draper, you can just break the Law of Demeter a little and go with a plain object that requires accessing the instance variables through the presenter:

class PostPresenter
  attr_reader :post, :current_user

  def initialize(post, current_user)
    @post = post
    @current_user = current_user
  end
end

Or you can use four other methods of decorating in Ruby.

Those four examples of decorating though didn't cover the simple delegates. So you can also use the Delegate class in Rails:

class PostPresenter
  attr_accessor :post
  attr_accessor :current_user

  delegate :title, :body, :author, :publication_date, to: :post
  def_delegators :@current_user, :name, to: :current_user

  def initialize(post, current_user)
    @post = post
    @current_user = current_user
  end
end

Or in plain Ruby, use Forwardable:

require 'forwardable'

class PostPresenter
  extend Forwardable
  def_delegators :@post, :title, :body, :author, :publication_date
  def_delegators :@current_user, :name

  def initialize(post, current_user)
    @post = post
    @current_user = current_user
  end
end

Read more about delegation in Ruby.

Dependency Inversion (the D in SOLID) used for testing

Now that I don't have logic embedded within my view, I don't even need to unit test them. I should still have my controller verify that the proper template was rendered and I need some basic level of integration testing but overall, I can skip view rendering.

To test my "RABL", I actually test my presenter. To test my presenter, I use Dependency Injection to supply my presenter with the objects I need it to test with.

Gregory Brown talks about Dependency Inversion in Issue #23: SOLID Design Principles of Ruby Best Practices. The problem with his examples and my example above is we're actually demonstrating Dependency Injection and not Inversion. An article on RubySource titled SOLID Ruby: Dependency Inversion Principle does a better job of explaining the differences between the two. It basically boils down to initializing the object with your dependencies versus calling methods with your dependencies passed in. Either way, it's the same damn thing. You're abstracting the interface between different implementations which makes it easier to change and test.

Test Doubles (Stubs and Mocks)

This is ultimately the entire point of this article. You have successfully segregated your dependency on multiple instance variables, random helper methods and anything else floating around in your scopes. When a view refers to foo or bar, you can look at the presenter to know exactly where that is coming from.

It also makes it possible to use stubs and mocks to easily swap out parts of the presenter for testing. You don't have to actually have a current_user, just mock(:current_user). You can also really speed up your tests by not using fixtures or factories to build out your user or post. Have post be Post.new with method stubs where needed.

RABL Issue #299 asks:

... we want to use current_user in our renderer ...
... it doesn't cover having, say, a current_user method in use ...

Now that's easy. Pass in whatever current_user you want to use to your presenter.

One last example...

Are you using the Timecop gem to manipulate your clock for testing? Just pass in a Time object:

class PostPublishPresenter
  include ActionView::Helpers::DateHelper

  def initialize(published_at)
    @published_at = published_at
  end

  def age(to_when = Time.now.utc)
    distance_of_time_in_words @published_at, to_when
  end
end

>> foo = PostPublishPresenter.new(Time.now.utc - 2.days)

>> foo.age
=> "2 days"

>> foo = PostPublishPresenter.new(Time.now.utc - 14.hours)

>> foo.age
=> "about 14 hours"

>> foo.age(Time.now.utc - 14.hours)
=> "less than a minute"