Fast specs and better design

Corey Haines first brought testing parts of Rails applications without loading Rails to my attention and excitement a few months ago, and I've been looking at how other developers do similar techniques. Having recently finished Objects on Rails and working through the Destroy All Software videos, I've been constantly looking at ways to make my Rails code better and faster to test. I believe these are related and that there is a lot of value in working to make the code easy to test and fast.

An example of some code I've written that doesn't rely on Rails is an observer. The observer inherits from ActiveRecord::Observer and would normally need Rails loaded to exercise its full functionality with the model it's observing. But if we try to keep Rails out and only load what we need, we can drive the design of the observer.

Here we save an activity, which is used as a generic Facebook-style activity stream item, after someone answers a question.

class AnswerObserver < ActiveRecord::Observer
  def after_create(answer)
    Activity.question_answered(answer)
  end
end

Here's the entire spec file.

require 'fast_spec/spec_helper'
stub_class 'ActiveRecord::Observer'
require 'app/models/observers/answer_observer'

class Activity; end

describe AnswerObserver do
  let(:observer) { AnswerObserver.new }

  describe "#after_create" do
    let(:answer) { stub }

    it "creates an activity" do
      Activity.should_receive(:question_answered).with(answer)
      observer.after_create(answer)
    end
  end
end

We do a few things here. The first is loading fast_spec/spec_helper.rb instead of the regular spec_helper.rb that loads the Rails environment. The fast spec helper requires a few utility files like awesome_print and logger, and a module_stub file that has a convenience method stolen from Avdi Grimm:

def stub_module(full_name)
  full_name.to_s.split(/::/).inject(Object) do |context, name|
    begin
      context.const_get(name)
    rescue NameError
      context.const_set(name, Module.new)
    end
  end
end

To quote Avdi:

This method uses #const_get to attempt to reference the given module. If the module is defined, or if calling #const_get causes it to be auto-loaded, the method does nothing more. But if #const_get fails to turn up the module, it defines an anonymous empty module to act as a placeholder.

Looking back at the spec file, we see a similar call to stub_class which does the same thing for classes. We also include the actual file of the code we're testing.

We create an empty Activity class, normally an ActiveRecord model, since we don't care what it's doing, just that it receives messages we're going to send to it. That's all this test does, really: we check that the Activity class gets a specific message passed to it with specific parameters.

Testing this way drives the code to be loosely coupled with objects this class needs to communicate with. We could have put all the activity action in the after_create callback and we'd be able to see what's going on, but we'd need to mock and stub a lot of interactions, or we'd have to go back to loading Rails, slowing our tests down. Plus we'd have a lot of code that has feature envy and would be difficult to maintain.

We know the test works because changing the functionality of the callback will break the test, and doing BDD will ensure that we're writing failing tests before we add functionality that could be skipped by our mocks. Of course, this problem exists with slower, more integration-like tests, so it's not like running through the full stack is a silver bullet.

There is a point where there can be too much mocking and stubbing, and I'll get to that in another post. For now, being able to run an entire suite of tests in less than half a second helps keep me in flow to write better software.