I recently wrote a Ruby client for Amazon Alexa’s APIs, and thought I’d pull out an example of nice, simple dependency injection to facilitate unit testing. Nothing revolutionary or complicated, just good practice.

The example is based around a UriSigner class, normally used by the calling code like this:

UriSigner.new(*credentials).sign_uri(uri)

The calling code doesn’t know or care that UriSigner depends on Base64, OpenSSL::Digest::SHA256 and OpenSSL::HMAC. The unit test for UriSigner, however, cares for two reasons.

Firstly, they’re external dependencies, and only need to be tested for correct usage, not correct implementation.

Secondly, these dependencies represent an encoder and a cryptographic hash function; they’re deterministic, but they return very opaque data which can make tests and their failure messages difficult to understand.

So instead of testing against magical (computed in advance) Base64 strings and HMAC hashes, I’ve used simple attr_writer dependency injectors:

The unit test can then inject MiniTest::Mock instances in place of the real Base64 and HMAC implementations, setting expected method calls and their return values:

As simple as that.

The same approach is used by the HTTP Client class to stub out actual HTTP calls via Net::HTTP. There’s great libraries like VCR, WebMock and FakeWeb for handling this, but sometimes it’s easier to keep it lo-fi:

This kind of dependency injection is one of many basic techniques which aren’t fancy enough to get a lot of press, but go a long way to keeping your objects and tests in order.

Got any thoughts? Hit me up, I’m @pda on Twitter, where I generally write about this kind of thing.

Slow Rails startup time is the TDD killer.

paul@paulbookpro ~/project ⸩ time rspec spec/lib/method_hunting_delegator_spec.rb
..
Finished in 0.00078 seconds
2 examples, 0 failures
rspec spec/lib/method_hunting_delegator_spec.rb -f d  6.76s user 1.64s system 91% cpu 9.225 total

Holy crap, that’s 9 seconds of Rails startup, for 0.00078 seconds worth of RSpec. And this class/test doesn’t even use Rails! We can do better.

The culprit? That require "spec_helper" at the top of every spec file which loads the entire of Rails:

# This file is copied to spec/ when you run 'rails generate rspec:install'
ENV["RAILS_ENV"] ||= 'test'
require File.expand_path("../../config/environment", __FILE__)
require 'rspec/rails'
require 'rspec/autorun'
# etc ...

There’s a few ways to deal with this, each with their own pitfalls. After trying many approaches, I’ve settled on a tiered RSpec initializer (spec_helper.rb and friends) which I can choose when invoking RSpec.

All the spec files still require "spec_helper", but it looks more like this:

Which means we can select different initializers using the SPEC environment variable. The following spec_helper_unit.rb is perfect for the method_hunting_delegator_spec which took 9 seconds earlier, because there’s no dependencies on Rails.

The result?

paul@paulbookpro ~/project ⸩ time SPEC=unit rspec spec/lib/method_hunting_delegator_spec.rb
..
Finished in 0.00079 seconds
2 examples, 0 failures
SPEC=unit rspec spec/lib/method_hunting_delegator_spec.rb  0.81s user 0.08s system 99% cpu 0.890 total

Under a second (0.890) is much more like it, and we still get class autoloading provided by ActiveSupport. I use this mode for just about everything except subclasses of Rails components, and those I keep a slim as possible. Moving logic into SOLID classes is something you’ll benefit from anyway, and these faster tests provide extra incentive. This example was a spec for a standalone class living in RAILS_ROOT/lib/ but I use it for all sorts of classes under app/models/, app/presenters/, app/forms/ etc.

But this zero-Rails initializer doesn’t help with testing your ORM-subclasses (we’ll begrudgingly call them “models”) which depend on ActiveRecord:

paul@paulbookpro ~/project ⸩ SPEC=unit rspec spec/models/book_spec.rb
/Users/paul/project/app/models/book.rb:4:in `<top (required)>': uninitialized constant ActiveRecord (NameError)

And having tasted sub-second tests, 12 seconds is clearly unacceptable:

paul@paulbookpro ~/project ⸩ time rspec spec/models/book_spec.rb
.................
Finished in 0.67016 seconds
17 examples, 0 failures
rspec spec/models/book_spec.rb  8.08s user 1.85s system 78% cpu 12.698 total

But if you write your classes carefully, they don’t need to depend on much from Rails except ActiveRecord. So let’s write a spec_helper which loads & configures ActiveRecord, plus a few other bits and pieces useful for testing database-persisted models.

You’ll have to excuse the Devise hackery; it was the one component tightly coupled into a model (User), and like most Rails app, that particular model is at the center of the whole relationship graph. Perhaps there’s a better solution, but this got me fast model tests for all but the user_spec itself.

Lets run that model spec again, this time boosted by SPEC=model:

paul@paulbookpro ~/project ⸩ time SPEC=model rspec spec/models/book_spec.rb
.................
Finished in 0.58512 seconds
17 examples, 0 failures
SPEC=model rspec spec/models/book_spec.rb  6.50s user 0.21s system 98% cpu 6.844 total

Note that your model classes can still depend on external gems, but they’ll need to e.g. require "money" at the top. I suspect this explicit declaration of dependency isn’t a bad idea anyway.

Of course, there’s always going to be specs which depend on the whole stack, such as acceptance tests. For those, here’s the default spec_helper_full.rb; basically like the original spec_helper.rb:

God speed.

I was implementing search in a Ruby app where the results objects were instances of a mix of model classes. Each one had what could be considered a title and a description, but the method names were inconsistent.

Wrapping each result in a SearchResult decorator to normalize the interface seemed like a good idea. Ruby provides an abstract Delegator and a concrete SimpleDelegator which gets most of the way there.

To normalize the method interface, I wrote an extension of SimpleDelegator with a #hunt_and_call(*candidates) method, which finds and calls the first method of the candidate list which the delegate responds to.

Here’s the example calling code:

And the MethodHuntingDelegator implementation:

And of course:

Completely insane? Useful enough for a gem? Better way of going about it? Give me hell in the gist comments.