Posts tagged with rails

The Right Way to Code DCI in Ruby

Many articles found in the Ruby community largely oversimplify the use of DCI. These articles, including my own, highlight how DCI injects Roles into objects at runtime, the essence of the DCI architecture. Many posts regard DCI in the following way:

class User; end # Data
module Runner # Role
  def run
    ...
  end
end

user = User.new # Context
user.extend Runner
user.run

There's a few flaws with oversimpilfied examples like this. First, it reads "this is how to do DCI". DCI is far more than just extending objects. Second, it highlights #extend as the go-to means of adding methods to objects at runtime. In this article, I would like to specifically address the former issue: DCI beyond just extending objects. A followup post will contain a comparison of techniques to inject Roles into objects using #extend and otherwise.

DCI (Data-Context-Interaction)

As stated previously, DCI is about much more than just extending objects at runtime. It's about capturing the end user's mental model and reconstructing that into maintainable code. It's an outside → in approach, similar to BDD, where we regard the user interaction first and the data model second. The outside → in approach is one of the reasons I love the architecture; it fits well into a BDD style, which further promotes testability.

The important thing to know about DCI is that it's about more than just code. It's about process and people. It starts with principles behind Agile and Lean and extends those into code. The real benefit of following DCI is that it plays nicely with Agile and Lean. It's about code maintainability, responding to change, and decoupling what the system does (it's functionality) from what the system is (it's data model).

I'll take a behavior-driven approach to implementing DCI within a Rails app, starting with the Interactions and moving to the Data model. For the most part, I'm going to write code first then test. Of course, once you have a solid understanding of the components behind DCI, you can write tests first. I just don't feel test-first is a great way of explaining concepts.

User Stories

User stories are an important feature of DCI although not distinct to the architecture. They're the starting point of defining what the system does. One of the beauties of starting with user stories is that it fits well into an Agile process. Typically, we'll be given a story which defines our end-user feature. A simplified story might look like the following:

"As a user, I want to add a book to my cart."

At this point, we have a general idea of the feature we'll be implementing.

Aside: A more formal implementation of DCI would require turning a user story into a use case. The use case would then provide us with more clarification on the input, output, motivation, roles, etc.

Write Some Tests

We should have enough at this point to write an acceptance test for this feature. Let's use RSpec and Capybara:

spec/integration/add_to_cart_spec.rb

describe 'as a user' do
  it 'has a link to add the book to my cart' do
    @book = Book.new(:title => 'Lean Architecture')
    visit book_path(@book)
    page.should have_link('Add To Cart')
  end
end

In the spirit of BDD, we've started to identify how our domain model (our Data) will look. We know that Book will contain a title attribute. In the spirit of DCI, we've identified the Context for which this use case enacts and the Actors which play key parts. The Context is adding a book to the cart. The Actor we've identified is the User.

Realistically, we would add more tests to further cover this feature but the above suites us well for now.

The "Roles"

Actors play Roles. For this specific feature, we really only have one Actor, the User. The User plays the Role of a customer looking to add an item to their cart. Roles describe the algorithms used to define what the system does.

Let's code it up:

app/roles/customer.rb

module Customer
  def add_to_cart(book)
    self.cart << book
  end
end

Creating our Customer Role has helped tease out more information about our Data model, the User. We now know we'll need a #cart method on any Data objects which plays the Customer Role.

The Customer role defined above doesn't expose much about what #cart is. One design decision I made ahead of time, for the sake of simplicity, is to assume the cart will be stored in the database instead of the sesssion. The #cart method defined on any Actor playing the Customer Role should not be an elaborate implementation of a cart. I merely assume a simple association.

Roles also play nicely with polymorphism. The Customer Role could be played by any object who responds to the #cart method. The Role itself never knows what type of object it will augment, leaving that decision up to the Context.

Write Some Tests

Let's jump back into testing mode and write some tests around our newly created Role.

spec/roles/customer_spec.rb

describe Customer do
  let(:user) { User.new }
  let(:book) { Book.new }

  before do
    user.extend Customer
  end

  describe '#add_to_cart' do
    it 'puts the book in the cart' do
      user.add_to_cart(book)
      user.cart.should include(book)
    end
  end
end

The above test code also expresses how we will be using this Role, the Customer, within a given Context, adding a book to the cart. This makes the segway into actually writing the Context dead simple.

The "Context"

In DCI, the Context is the environment for which Data objects execute their Roles. There is always at least one Context for every one user story. Depending on the complexity of the user story, there may be more than one Context, possibly necessitating a story break-down. The goal of the Context is to connect Roles (what the system does) to Data objects (what the system is).

At this point, we know the Role we'll be using, the Customer, and we have a strong idea about the Data object we'll be augmenting, the User.

Let's code it up:

app/contexts/add_to_cart_context.rb

class AddToCartContext
  attr_reader :user, :book

  def self.call(user, book)
    AddToCartContext.new(user, book).call
  end

  def initialize(user, book)
    @user, @book = user, book
    @user.extend Customer
  end

  def call
    @user.add_to_cart(@book)
  end
end

Update: Jim Coplien's implementation of Contexts uses AddToCartContext#execute as the context trigger. To support Ruby idioms, procs and lambdas, the examples have been changed to use AddToCartContext#call.

There's a few key points to note:

  • A Context is defined as a class. The act of instantiating the class and calling it's #call method is known as triggering.
  • Having the class method AddToCartContext.call is simply a convenience method to aid in triggering.
  • The essence of DCI is in @user.extend Customer. Augmenting Data objects with Roles ad hoc is what allows for strong decoupling. There're a million ways to inject Roles into objects, #extend being one. In a followup article, I'll address other ways in which this can be accomplished.
  • Passing user and book objects to the Context can lead to naming collisions on Role methods. To help alleviate this, it would be acceptable to pass user_id and book_id into the Context and allow the Context to instantiate the associated objects.
  • A Context should expose the Actors for which it is enabling. In this case, attr_reader is used to expose @user and @book. @book isn't an Actor in this Context, however it's exposed for completeness.
  • Most noteably: You should rarely have to (impossibly) #unextend a Role from an object. A Data object will usually only play one Role at a time in a given Context. There should only be one Context per use case (emphasis: per use case, not user story). Therefore, we should rarely need to remove functionality or introduce naming collisions. In DCI, it is acceptable to inject multiple Roles into an object within a given Context. So the problem of naming collisions still resides but should rarely occur.

Write Some Tests

I'm generally not a huge proponent of mocking and stubbing but I think it's appropriate in the case of Contexts because we've already tested running code in our Role specs. At this point we're just testing the integration.

spec/contexts/add_to_cart_context_spec.rb

describe AddToCartContext do
  let(:user) { User.new }
  let(:book) { Book.new }

  it 'adds the book to the users cart' do
    context = AddToCartContext.new(user, book)
    context.user.should_recieve(:add_to_cart).with(context.book)
    context.call
  end
end

The main goal of the above code is to make sure we're calling the #add_to_cart method with the correct arguments. We do this by setting the expectation that the user Actor within the AddToCartContext should have it's #add_to_cart method called with book as an argument.

There's not much more to DCI. We've covered the Interaction between objects and the Context for which they interact. The important code has already been written. The only thing left is the dumb Data.

The "Data"

Data should be slim. A good rule of thumb is to never define methods on your models. This is not always the case. Better put: "Data object interfaces are simple and minimal: just enough to capture the domain properties, but without operations that are unique to any particular scenario" (Lean Architecture). The Data should really only consist of persistence-level methods, never how the persisted data is used. Let's look at the Book model for which we've already teased out the basic attributes.

class Book < ActiveRecord::Base
  validates :title, :presence => true
end

No methods. Just class-level definitions of persistence, association and data validation. The ways in which Book is used should not be a concern of the Book model. We could write some tests around the model, and we probably should. Testing validations and associations is fairly standard and I won't cover them here.

Keep your Data dumb.

Fitting Into Rails

There's not a whole lot to be said about fitting the above code into Rails. Simply put, we trigger our Context within the Controller.

app/controllers/book_controller.rb

class BookController < ApplicationController
  def add_to_cart
    AddToCartContext.call(current_user, Book.find(params[:id]))
  end
end

Here's a diagram illustrating how DCI compliments Rails MVC. The Context becomes a gateway between the user interface and the data model.

MVC + DCI

What We've Done

The following could warrant it's own article, but I want to briefly look at some of the benefits of structuring code with DCI.

  • We've highly decoupled the functionality of the system from how the data is actually stored. This gives us the added benefit of compression and easy polymorphism.
  • We've created readable code. It's easy to reason about the code both by the filenames and the algorithms within. It's all very well organized. See Uncle Bob's gripe about file-level readability.
  • Our Data model, what the system is, can remain stable while we progress and refactor Roles, what the system does.
  • We've come closer to representing the end-user mental model. This is the primary goal of MVC, something that has been skewed over time.

Yes, we're adding yet another layer of complexity. We have to keep track of Contexts and Roles on top of our traditional MVC. Contexts, specifically, exhibit more code. We've introduce a little more overhead. However, with this overhead comes a large degree of prosperity. As a developer or team of developers, it's your descretion on whether these benefits could resolve your business and engineering ailments.

Final Words

Problems with DCI exist as well. First, it requires a large paradigm shift. It's designed to compliment MVC (Model-View-Controller) so it fits well into Rails but it requires you to move all your code outside the controller and model. As we all know, the Rails community has a fetish for putting code in models and controllers. The paradigm shift is large, something that would require a large refactor for some apps. However, DCI could probably be refactored in on a case-by-case basis allowing apps to gradually shift from "fat models, skinny controllers" to DCI. Second, it potentially carries performance degradations, due to the fact that objects are extended ad hoc.

The main benefit of DCI in relation to the Ruby community is that it provides a structure for which to discuss maintainable code. There's been a lot of recent discussion in the vein of "'fat models, skinny controllers is bad'; don't put code in your controller OR your model, put it elsewhere." The problem is we're lacking guidance for where our code should live and how it should be structured. We don't want it in the model, we don't want it in the controller, and we certainly don't want it in the view. For most, adhering to these requirements leads to confusion, overengineering, and a general lack of consistency. DCI gives us a blueprint to break the Rails mold and create maintainable, testable, decoupled code.

Aside: There's been other work in this area. Avdi Grimm has a phenominal book called Objects on Rails which proposes alternative solutions.

Happy architecting!

This article is translated to Serbo-Croatian.

Further Reading

DCI: The King of the Open/Closed Principle
DCI With Ruby Refinements
DCI Role Injection in Ruby
Benchmarking DCI in Ruby

Posted by Mike Pack on 01/24/2012 at 12:58PM

Tags: architecture, rails, ruby, dci, testing


Benchmarking DCI in Ruby

I've recently become quite intrigued with the concepts behind DCI (Data Context and Interaction). I won't go too in depth about what DCI is or why you might use it, that's been discussed many times elsewhere. In short, DCI is an architecture which allows us to delineate our domain objects from the actual functions they perform. It mixes-in Roles (functionality) into our Data component when and only when that functionality it needed; when in Context. Most of the value DCI brings to the table derives from the way it forces you to abstract out behavior into testable modules.

What I'd like to do is take a look at the performance implications of using DCI in Ruby applications.

I think it should be said upfront that this is purely academic and may have minimal bearing within the ecosystem of a complex app. For this sake, I won't try to draw any vast conclusions.

How to use DCI in Ruby

DCI can be used in Ruby by augmenting your objects with Roles at runtime so that the necessary interactions are available to that object.

class User
  ...
end

module Runner
  def run
    ...
  end
end
 
user = User.new
user.extend Runner
user.run 

In more traditional, idiomatic Ruby you would normally just include the module while defining the class:

class User
  include Runner
  ...
end

user = User.new
user.run 

Hypothesis

Since every #extend called carries some memory and processing implications, lately I've been wondering if, while using DCI, we could be incurring a performance hit when extending many objects ad hoc. I decided to profile this to understand if we could be blindly degrading performance and whether there are optimization techniques I should be aware of.

My process involves taking the most simplified example (shown in the above snippets) and benchmarking the traditional approach against the DCI-inclined approach.

I'm running these benchmarks on a MacBook Pro - 2.2 GHz - 8 GB memory.

The Runner Module

Here's the Runner module used in the following examples. It's just one method that does some arbitrary calculation.

runner.rb

module Runner
  def run
    Math.tan(Math::PI / 4)
  end
end

Ruby Benchmark

Using Ruby's Benchmark library, we can extrapolate the amount of time taken for these processes to execute. First, we'll benchmark the traditional, idiomatic Ruby way: using an include to augment the class.

include_bm.rb

require 'benchmark'
require './runner'

class IncludeUser
  include Runner
end

Benchmark.bm do |bench|
  3.times do
    bench.report('include') do
      1000000.times do
        user = IncludeUser.new
        user.run
      end
    end
  end
end
$ ruby include_bm.rb
         user       system     total       real
include  0.500000   0.000000   0.500000 (  0.497114)
include  0.500000   0.000000   0.500000 (  0.497363)
include  0.490000   0.000000   0.490000 (  0.497342)

The results of this benchmark tell us that executing 1 million "run" operations results in roughly 0.5 seconds.

Let's look at how this compares to the DCI implementation.

dci_bm.rb

require 'benchmark'
require './runner'

class DCIUser; end

Benchmark.bm do |bench|
  3.times do
    bench.report('DCI') do
      1000000.times do
        user = DCIUser.new
        user.extend Runner
        user.run
      end
    end
  end
end
$ ruby dci_bm.rb
     user       system     total       real
DCI  8.430000   0.000000   8.430000 (  8.429382)
DCI  8.490000   0.010000   8.500000 (  8.486804)
DCI  8.450000   0.010000   8.460000 (  8.447363)

Quite a difference! It's probably safe to say at this point that calling extend 1 million times is a lot less performant than including the module one time as the class is defined. The reasoning is simple. Including the module once injects it in the user objects' lookup hierarchy. When the run method is called, the hierarchy is traversed and the method is fetched. In the traditional (include) approach, the module never leaves or reenters the hierarchy after it's been defined. Conversely, in DCI, the module enters the hierarchy each time extend is called.

Let's profile these two approaches and discover why they're so different.

perftools.rb

Assuming the same class/module structure as above, let's use perftools.rb to profile their execution. Using perftools.rb is a two-step process. First, generate the profile: a summary of where the code is spending it's time. Second, display the profile in the designated format. To visualize the components, we'll genereate graphs using the GIF format. You'll need the dot tool in order to generate graphs. Check out this presentation for more info on using perftools.rb.

Let's first observe the traditional approach:

include_profile.rb

require 'perftools'
require './runner'

class IncludeUser
  include Runner
end

PerfTools::CpuProfiler.start('/tmp/include_profile') do
  1000000.times do
    user = IncludeUser.new
    user.run
  end
end
$ ruby include_profile.rb
$ pprof.rb --gif /tmp/include_profile > include_profile.gif

include_profile.gif

The above graph tell us that most of the execution time is happening in the iteration of our test loop. Barely any time is spent creating the objects or executing the arbitrary math calculation. More info on reading the pprof.rb output can be found here.

Now let's take a look at the DCI approach:

dci_profile.rb

require 'perftools'
require './runner'

class DCIUser; end

PerfTools::CpuProfiler.start('/tmp/dci_profile') do
  1000000.times do
    user = DCIUser.new
    user.extend Runner
    user.run
  end
end
$ ruby dci_profile.rb
$ pprof.rb --gif /tmp/dci_profile > dci_profile.gif

dci_profile.gif

The above results tell us that almost half the time is spent extending objects at runtime through Module#extend_object. In this example, the time spent iterating over our test case is dwarfed against the time taken to extend objects. So, after profiling we can verify that extending the object is indeed taking up most of our time.

ObjectSpace.count_objects

Let's compare how the number of objects in memory stack up with the two implementations. Ruby 1.9 provides us with the ObjectSpace.count_objects method to inspect all objects currently initialized in memory. It's important to turn off garbage collection as it may be invoked mid-test, skewing the results. Here is the module used to inspect the number of objects currently in memory. It's a modified version of Aaron Patterson's implementation.

allocation.rb

module Allocation
  def self.count
    GC.disable
    before = ObjectSpace.count_objects
    yield
    after = ObjectSpace.count_objects
    after.each { |k,v| after[k] = v - before[k] }
    GC.enable
    after
  end
end

This method turns off the garbage collector, records the number of objects pre-benchmark, runs the benchmark, records the number of objects post-benchmark, then compiles the difference between the two. Let's gather more information by exracting object allocations.

include_space.rb

require './runner'
require './allocation'

class IncludeUser
  include Runner
end

p(Allocation.count do
  1000000.times do
    user = IncludeUser.new
    user.run
  end
end)
$ ruby include_space.rb
{:TOTAL=>2995684, :FREE=>-4344, :T_OBJECT=>1000000, :T_CLASS=>0, :T_MODULE=>0, :T_FLOAT=>2000000, :T_STRING=>27, :T_REGEXP=>0, :T_ARRAY=>0, :T_HASH=>1, :T_BIGNUM=>0, :T_FILE=>0, :T_DATA=>0, :T_COMPLEX=>0, :T_NODE=>0, :T_ICLASS=>0}

Most of the keys in the printed hash refer to Ruby types. The ones we're interested in are :TOTAL, :T_CLASS, :T_ICLASS. The meaning of these keys isn't very well documented but the Ruby source hints at them. Here's my understanding:

:TOTAL is the total number of objects present in memory.
:T_CLASS is the total number of classes that have been declared (in memory).
:T_ICLASS is the total number of internal use classes, or iClasses, used when modules are mixed in.

The DCI approach:

dci_space.rb

require './runner'
require './allocation'

class DCIUser; end

p(Allocation.count do
  1000000.times do
    user = DCIUser.new
    user.extend Runner
    user.run
  end
end)
$ ruby dci_space.rb
{:TOTAL=>4995536, :FREE=>-4492, :T_OBJECT=>1000000, :T_CLASS=>1000000, :T_MODULE=>0, :T_FLOAT=>2000000, :T_STRING=>27, :T_REGEXP=>0, :T_ARRAY=>0, :T_HASH=>1, :T_BIGNUM=>0, :T_FILE=>0, :T_DATA=>0, :T_COMPLEX=>0, :T_NODE=>0, :T_ICLASS=>1000000}

Let's looks at the significant differences between the two:

# Include
{:TOTAL=>2995684, :FREE=>-4344, :T_CLASS=>0, :T_ICLASS=>0}

# DCI
{:TOTAL=>4995536, :FREE=>-4492, :T_CLASS=>1000000, :T_ICLASS=>1000000}

These results expose a few facts about the two implementations.

  • The DCI approach uses roughly 40% more objects in memory.
  • The DCI approach initializes many more classes and internal (mixed-in) classes.

Wrapping It Up

As I said at the beginning, it's quite hard to determine how, if at all, using DCI will affect the performance of a real world application. Certainly, a single web request will never invoke 1 million inclusions of a module at runtime.

What this does show us is that Ruby is not optimized for this architecture. In idiomatic Ruby, modules are usually included during the class definition. It's possible that languages, like Scala, with built-in tools to extend objects ad hoc perform better than Ruby. Scala's traits provide high-level support for this type of functionality and are optimized for use with DCI.

I'm still quite interested in DCI. Specifically, in optimizations for Ruby. I'm also quite interested in running these benchmarks against a production app, something that'll just have to wait.

All the code used here can be found on github.

Happy benchmarking!

Posted by Mike Pack on 01/17/2012 at 12:46PM

Tags: dci, ruby, rails, performance, benchmarking, profiling, architecture


Seeding data to your Rails tests with factory_girl

Many applications rely on seed data as a basic set of information to get their app off the ground and functional. Seed data to an application is like wheels to a car. Without the wheels, your car won't function.

The concept of seed data works great when thinking about how the applications functions in the real world. However, when running tests against your application, it poses the question of "Well, where did the data come from?" It's like the philosophical question of the chicken or the egg.

You have a test database ready to rock. You might even use rake db:test:prepare to get it set up. Now you run your specs and uh oh! they fail because your seed data doesn't exist. Lets review some solutions to this problem.

Just seed the test database!

Stop with all the fuss and just seed the test database. rake db:seed and your test will be happy.

The problem with seeding your test database is you're now creating a dependency, and a large one at that. Every time you plan to execute your tests on a fresh database, you need the additional step of seeding. Not only that, but your tests are written to rely on an outside source to provide the seed data.

Your tests should viably run in a bubble. Give them an empty database and some application code and they should run to completion. Seeding the test database is an anti-pattern, in my opinion.

Enter the seed data when appropriate

Say you have a User model and after the user is created, they are given a gold star award for signing up. Assume your awards are stored in an awards table with an Award model and are seeded into your application.

When your tests rely on seed data, they might look like this (using factory_girl):

describe 'after signing up as a user'
  before do
    @user = Factory(:user)
  end

  it 'it gives me a gold star award' do
    @user.has_award('gold_star')
  end
end

In the above example, we're relying upon the fact that the gold star award has already been seeded into the database.

When your tests don't rely on seed data, they might look like this:

describe 'after signing up as a user'
  before do
    # Add the gold star award
    Factory(:award, :name => 'gold_star')
    @user = Factory(:user)
  end

  it 'it gives me a gold star award' do
    @user.has_award('gold_star')
  end
end

Now you've successfully removed the dependency on the seed data and brought the responsibility of database inadequacies to the test suite. This is a great step forward in refactoring the tests but now, every time you create a User with Factory(:user), you need to add the gold star award to the database. Lets fix this up.

Let factory_girl do the heavy lifting

factory_girl comes built in with a few callbacks. The available callbacks are #after_build, #after_create and #after_stub. Read more about using factory_girl callbacks on thoughtbot's blog (updated docs).

So, in lieu of adding the gold star award right before creating a User in every one of our tests, lets use a callback when creating a User.

FactoryGirl.define do
factory :user do
  name "Mike Pack"
  after_build do |user|
     Factory(:award, :name => 'gold_star') unless Award.find_by_name('gold_star').present?
  end
end
end 

Now, our tests can be as clean as possible:

describe 'after signing up as a user'
  before do
   @user = Factory(:user)
  end

  it 'it gives me a gold star award' do
    @user.has_award('gold_star')
  end
end

Every time a user is created with factory_girl, the dependency is built using the after_build callback.

Happy testing!

Posted by Mike Pack on 12/22/2011 at 03:01PM

Tags: rails, rspec, testing, factory_girl


Conditional Indexing with Sunspot

One of my favorite new features of Sunspot 1.3 is the ability to conditionally index an instance of a model based on anything that returns a boolean.

Say I have a Post model to store my blog posts. We want to index only blog posts which are published so users aren't searching on unpublished posts. The syntax looks as follows:

class Post < ActiveRecord::Base
  attr_accessible :title, :content, :published, :external_source

  searchable :if => :published do
    text :title
    text :content
  end
end

Pretty nice. But let's elaborate a little. Say we want to index only published posts but we don't want to index posts where content comes from an external source.

searchable :if => :published, :unless => :external_source do
  ...
end

Let's flip things around. What if want to index only published blog posts which come from external sources? Supply an array.

searchable :if => [:published, :external_source] do
  ...
end 

What if the conditions for indexing are more complex than a simple boolean method on our model? Supply a proc.

searchable :if => proc { |post| post.content.size > 0 } do
  ...
end 

Indexing with sunspot just got a whole lot easier.

Happy indexing!

Posted by Mike Pack on 11/16/2011 at 02:32PM

Tags: sunspot, search, indexing, rails


Stubbing Internal Methods and Rails Associations with RSpec

Occasionally, times arise where you would like to unit test the inner workings of a method. As a disclaimer, I don't recommend it because tests should generally be behavior driven. Tests should treat your methods as black boxes; you put something in, you get something out. How it works internally shouldn't really matter. However, if you would like to test the inner workings of your methods there's a number of ways to do so with pure Ruby including :send, :instance_variable_get and others. Testing the innards feels dirty no matter which way you spin it but I like to at least do it with RSpec.

Why Test the Internals?

Lets say you have a method that does some expensive lookup:

class Library < ActiveRecord::Base
  include ExpensiveQueries

  att_accessor :books
  def books
    @books ||= expensive_query # The expensive query takes 5 seconds
  end
end

The above example should be familiar, you plan to perform something Ruby or database expensive and you would like to cache the result in an instance variable so that all subsequent calls to that method draw from the instance variable.

If you were taking a TDD approach, you wouldn't have this class already written. In that case, you know you'll be performing something very expensive and you want to ensure that method caches the result. How do you test this without knowing the internals of the method?

Stubbing with RSpec

Let's say you're writing your tests before you write the above class. You could use RSpec's stubbing library to ensure your method is caching it's result.

describe Library do
  describe '#books' do
    it 'caches the result' do
# Assume some books get associated upon creation
      @library = Library.create!

# 5 seconds for this call
      the_books = @library.books

# Stub out the expensive_query method so it raises an error
      @library.stub(:expensive_query) { raise 'Should not execute' }

# If the value was cached, expensive_query shouldn't be called
      lambda { @library.books }.should_not raise_error
    end
  end
end

The key component here is the @library.stub call. This is also where we're breaking the black box, behavior driven test idiom. We assume at this line that we know there will be a method call internally named expensive_query. This test is also brittle because if expensive_query ever changes it's name to really_expensive_query, our test will break even though the functionality of our method remains the same.

Stubbing Rails Associations

What if your expensive_query is really an ActiveRecord association? So, let's say your Library class looks more like the following:

class Library < ActiveRecord::Base
  has_many books

  att_accessor :authors
  def authors
    @authors ||= books.authors # The expensive query takes 5 seconds
  end
end

You could use the nifty stub_chain method provided by RSpec to stub the books.authors method and ensure it only gets called once.

describe Library do
  describe '#books' do
    it 'caches the result' do
# Assume some books and authors get associated upon creation
      @library = Library.create!

# 5 seconds for this call
      the_authors = @library.authors

# Stub out the books.authors association so it raises an error
      @library.stub_chain(:books, :authors) { raise 'Should not execute' }

# If the value was cached, books.authors shouldn't be called
      lambda { @library.authors }.should_not raise_error
    end
  end
end

Arguments to stub_chain represent the associations used. stub_chain could also be used to stub out additional methods which get called within the chain.

Happy stubbing!

Posted by Mike Pack on 10/07/2011 at 11:20AM

Tags: rails, rspec, stubbing


Managing Devise's current_user, current_admin and current_troll with CanCan

CanCan is awesome. It lets you manage user abilities easily and provides ways to define complex scenarios. I highly recommend using it for anyone who has more than one user type (like Troll).

Devise is great for authentication. When you have more than one user type as distinct classes, Devise will create current_* to be used in your controllers and views. So, User class corresponds to current_user. Admin class corresponds to current_admin. Troll class (used to identify Trolls under your application's bridge) corresponds to current_troll.

The problem

CanCan doesn't work with current_admin and current_troll out-of-the-box. It assumes that current_user is defined and current_user's abilities are defined in the Ability class. What if you want to break this paradigm? It turns out CanCan makes this pretty easy. Here are the current_user and Ability assumptions I am referring to:

def current_ability
@current_ability ||= ::Ability.new(current_user)
end

CanCan defines current_ability on your controller. This grabs an instance of the Ability class for the current user. So it assumes that you have current_user set and you have an Ability class defined. When your user types get more complex than what can be handled by one User model, it's time to make some changes.

Working with numerous Ability classes

Up front, your project might not require many different user types that vary greatly from one another. It might make sense to use Rail's nifty STI (Single Table Inheritance) and add all your abilities to one class. This can be nice in some respect. For instance, all users, no matter which type, can be reference by current_user.

When your user types get too complex to use one User model, your Ability class is too complex as well. In it's most simple form, say you have an Ability class that looks as follows:

class Ability
  include CanCan::Ability
 
  def initialize(user)
    user
||= User.new # guest user (not logged in)
    if user.is_a? Admin
      # Admin abilities
    elsif user.is_a? Troll
      # Troll abilities
    elsif user.new_record?
      # Guest abilities
    else
      # Basic user abilities
    end
  end
end

This structure gives you some flexibility in how you define your abilities but it's on it's way to Maintenance Hell, a deep dark place with no exit.

It would be best to define your abilities in different classes. Here we define UserAbility, AdminAbility, TrollAbility, and GuestAbility.

class UserAbility
  include CanCan::Ability
  def initialize(user)
    # Basic user abilities
  end
end
class AdminAbility
  include CanCan::Ability

  def initialize
    # Admin abilities
  end
end
class TrollAbility
  include CanCan::Ability
  def initialize(user)
   # Troll abilities
end
end
class GuestAbility
  include CanCan::Ability

  def initialize
    # Guest abilities
  end
end

Keep in mind that if your abilities are a subset of another user's abilities, you can inherit from other ability class. So in our case a Troll is a user who lives under a bridge. We don't want the Trolls to talk, so we limit their ability to post comments. Otherwise, they can do everything a User can do.

class TrollAbility < UserAbility
  def initialize(user)
    super(user)
    cannot :create, Comment     # More Troll abilities   end
end

Hooking up the Ability classes

When you use CanCan's "can? :create, Comment" method, it refers to current_ability to determine whether the given abilities include :create, Comment.

Since CanCan makes the assumption we're working with current_user and strictly Ability, we need to extend the built-in functionality. We do this by instantiating the new Ability classes based on the current user type (defined by Devise). CanCan has a brief wiki post on this topic.

def current_ability
  @current_ability ||= case
                       when current_user
                         UserAbility.new(current_user)
                       when current_admin
                         AdminAbility.new                        when current_troll                          TrollAbility.new(current_troll)
                       else
                         GuestAbility.new
                       end
end

 Now, when CanCan needs to check abilities (when you call "can? :create, Comment"), your current_ability method will return the appropriate Ability class.

Happy CanCaning!

 

Posted by Mike Pack on 07/21/2011 at 09:01PM

Tags: rails, devise, cancan


Testing Mobile Rails Apps with Capybara

Every web app should have a mobile version and every mobile version should be tested. Testing mobile web apps shouldn't be any more painful than testing desktop apps with the assumption that you're still serving up HTML, CSS, and JavaScript.

My Setup

For mobile detection, I use ActiveDevice, a User Agent sniffing library and some helper methods. While User Agent sniffing isn't the best approach for client-side (use feature detection with something like Modernizr), it's a reliable way to detect mobile devices in Rails.

For acceptance testing that doesn't need to be readily demonstrated to stakeholders, I use straight up Capybara with RSpec. Sometimes I use Steak.

The Pain of Testing Mobile

It's difficult to test mobile web apps because Capybara's default drivers are all desktop User Agents. You could acquire Capybara-iphone, but this solution didn't produce the expected results for me. I was given my mobile views but not my mobile layout. Plus, all this driver does is reset the User Agent for the Rack-Test driver. Further, what if you use Selenium for your default driver?

Platformatec wrote a nice blog post about mobile testing with user agents. The problem is it relies on Selenium. I was seeking a more concrete, driver agnostic way to serve up mobile views to my tests.

How I Test Mobile

This is a simple, straightforward way to invoke your mobile app from within your tests. I'm not convinced it's the most elegant way, but it's clean and simple.

Setup Your Application

In your ApplicationController, give some support for changing over to  your mobile app. This will also come in handy when you want to work on and test your mobile app on your desktop browser.

app/controllers/application_controller.rb

@@format = :html
cattr_accessor :format

Here we simply add a class attribute accessor with a default of :html. This will allow us to say ApplicationController.format

Next we need to add a before filter which will set the desired format upon each request.

app/controllers/application_controller.rb

before_filter :establish_format

private
def establish_format
  # If the request is from a true mobile device, don't set the format
request.format = self.format unless request.format == :mobile
end

Here's what a sample Rails 3 ApplicationController would look like:

app/controllers/application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  include ActiveDevice

  # force the mobile version for development:
  #@@format = :mobile
  @@format = :html
  cattr_accessor :format
  before_filter :establish_format

private
  def establish_format
# If the request is from a true mobile device, don't set the format
request.format = self.format unless request.format == :mobile
end
end

Setup Your Tests

Now, in your tests, you can set the desired format for your application.

spec/integration/mobile/some_spec.rb

require 'spec_helper'

describe 'on a mobile device' do
  before do
    ApplicationController.format = :mobile
  end

  after do
    ApplicationController.format = :html
  end

  describe 'as a guest on the home page' do
    before do
      visit root_path
    end

    it 'does what I want' do
      page.should do_what_i_want
    end
  end
end

By setting ApplicationController.format = :mobile, we force the application to render the mobile version of files, for instance: index.mobile.erb. Your application will be invoked, your before filter will be run, and your are serving the mobile app to your tests.

Note: You need to reset your format to :html after your mobile tests are run so that tests which follow this in your suite are run under the default format, :html.

Happy testing!

Posted by Mike Pack on 06/16/2011 at 11:05AM

Tags: rails, testing, rspec, capybara, mobile


Dynamically Requesting Facebook Permissions with OmniAuth

One of the major benefits of dynamically requesting the Facebook permissions is the increased rate of users who will allow you to access their account. Facebook puts it nicely, "There is a strong inverse correlation between the number of permissions your app requests and the number of users that will allow those permissions."

This solution uses OmniAuth to handle the authentication. The concept is simple. Ask the user to allow your application access to their most basic information (or the bare minimum your app needs). When they perform an action that requires more than the permissions they have currently allowed, redirect them to Facebook and ask for more permissions.

If you haven't set up OmniAuth, follow Ryan Bate's Railscasts, Part 1 and Part 2.

Configuring OmniAuth

OmniAuth expects you to configure your authentication schemes within your initializers.

config/initializers/omniauth.rb

Rails.application.config.middleware.use OmniAuth::Builder do
provider :facebook, ENV['FB_APP_ID'], ENV['FB_APP_SECRET']
end

Now, when you visit /auth/facebook, you will be redirected to Facebook and asked for basic permissions.

In order to permit your app to dynamically change the OmniAuth Stategy, you'll need a controller which has access to your OmniAuth Strategy. OmniAuth provides a pre-authorization setup hook to handle this. Update your omniauth.rb initializer to look like the following:

config/initializers/omniauth.rb

Rails.application.config.middleware.use OmniAuth::Builder do
provider :facebook, ENV['FB_APP_ID'], ENV['FB_APP_SECRET'], :setup => true
end

Now when you visit /auth/facebook you'll be redirected to /auth/facebook/setup. You should add a route for this:

config/routes.rb

match '/auth/facebook/setup', :to => 'facebook#setup'

Your Facebook controller with the setup action should look as follows:

app/controllers/facebook_controller.rb

class FacebookController < ApplicationController
  def setup
    request.env['omniauth.strategy'].options[:scope] = session[:fb_permissions]
    render :text => "Setup complete.", :status => 404
  end
end

Note: OmniAuth says, "we render a response with a 404 status to let OmniAuth know that it should continue on with the authentication flow."

Once your FacebookController#setup action has completed, OmniAuth will take it from there and process your request through to Facebook.

Dynamically Setting the Permissions

The appropriate code can be used like so:

app/controllers/some_controller.rb

session[:fb_permissions] = 'user_events'
redirect_to '/auth/facebook'

session[:fb_permissions] is the interface between your two controller actions: the one that wants to request more permissions (some_controller.rb) and the one that wants to modify your OmniAuth Strategy (facebook_controller.rb).

For reference, here's a list of available Facebook permissions you can use; comma deliminated.

That's it. Upon redirection, Facebook will ask to allow the new permissions, redirect back to your app, and you can now successfully make calls to the Facebook API (I use Koala to work with the Facebook API).

One Gotcha

One thing I ran into on OmniAuth 0.2.4 and Rails 3.0.7 is the OmniAuth Stategy which was available in request.env['omniauth.stategy']. If you have more than one provider in your OmniAuth::Builder DSL, request.env['omniauth.stategy'] will be set to the last entry in the DSL. If you have your initializer set up like the following:

Rails.application.config.middleware.use OmniAuth::Builder do
provider :facebook, ENV['FB_APP_ID'], ENV['FB_APP_SECRET'], :setup => true
provider :twitter, ENV['TWITTER_CONSUMER_KEY'], ENV['TWITTER_CONSUMER_SECRET']
end

request.env['omniauth.stategy'] will be set to #<OmniAuth::Strategies::Twitter>, not exactly what you want. Your Facebook stategy needs to be the last entry in the DSL, like so:

Rails.application.config.middleware.use OmniAuth::Builder do
provider :twitter, ENV['TWITTER_CONSUMER_KEY'], ENV['TWITTER_CONSUMER_SECRET']
provider :facebook, ENV['FB_APP_ID'], ENV['FB_APP_SECRET'], :setup => true
end

Happy Facebooking!

Posted by Mike Pack on 04/27/2011 at 10:51AM

Tags: ruby, rails, omniauth, facebook