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Testing POROs In Rails

A lot of smarter people than I have come up with the idea that moving your business logic out of framework based classes and into Plain Old Ruby Objects (POROs) is a Good Thing.

Being less tied to your framework makes upgrades easier. It also means you can test these objects without loading rails. That’s really the only way to get super fast unit tests. It’s also the only way to get true and isolated unit tests.

The problem is that even our PORO code will have some implicit dependencies on parts of rails. Some of these are Good Things that we don’t want to stamp out, others are just too annoying to work around in the PORO or its spec. This makes testing them in isolation difficult and awkward, which is a key indicator of a Bad Idea.

What we need is a compromising spec helper. One which brings in just enough rails so these niceties work as any rails developer might expect but not so much rails that it slows down the suite in any noticeable way. This lets our rails app be developed using common conventions and idioms but we also get a sick-fast Unit suite.

I’m figuring this out as I go, exploring through a toy application, but here’s what I’ve got so far:

spec/spec_helper.rb

# It's super convenient to type 10.hours or t.to_s(:db), we want to be 
# able to use these things freely in our PORO code.
require 'active_support/core_ext'

# Unless you have concerns about making inter-class dependencies 
# explicit, there's really no good reason to add requires all over the 
# place.
require 'active_support/dependencies'

# Yes, this is a bit of duplication you have to manage between here and 
# your application config, but I think it's worth it.
%w( lib app/models ).each do |dir|
  $LOAD_PATH << dir
  ActiveSupport::Dependencies.autoload_paths << dir
end

# Without fail, a PORO is going to reference an AR-based class. Your 
# PORO spec can (and should) stub all the behavior of that class, but 
# simply loading it will reference this constant.
module ActiveRecord
  class Base
    # You should also make no-ops for any class-level macros you use. 
    # Again, we should be stubbing as needed, this just allows the class 
    # definition to be sourced without error.
    class << self
      def belongs_to(*); end
      def has_many(*)  ; end
      # ...
    end
  end
end

11 Feb 2013, tagged with ruby