Living In the House That Rails Built

January 29, 2008

I wanted to share a snippet of code. This code will print a call stack to STDOUT every time a Ruby class definition is evaluated. It is particularly useful when you find that class constants are being mysteriously redefined.

class Foo
  puts "\nRequired from:\n #{Kernel.caller.join("\n ")}"
  # ...

What inspired me to write that code? Rails did. The key to writing Ruby on Rails is that you’re writing Ruby on Rails. You don’t follow the Rails best practices because they’re convenient. You follow the Rails best practices because your program won’t work unless you do. Just like trains, you stay on the track and everything is great. If you try to take your train off-track, then it’s gruesome enough to make the nightly news.

How did I derail my application such that I cared how and where a file was being required? I wrote a unit test that explicitly required a model object. Oops. Remember that the semantics of require is load-once based on the name. So:

require "foo"


require "models/foo"

are very different to require. Rails is super helpful and requires everything that it makes for you. So it requires models for you, even when you run your unit tests.

So take this code:

class Foo < ActiveRecord::Base

And then write a test for something that Rails didn’t generate (such as something in the lib directory like I did):

# Require some other stuff
require "foo"

class TestTruth < Test::Unit::TestCase
  def test_truth
    assert true

If you rake test you will get an error complaining that RAILS_IS_A_GHETTO was reinitialized, and that’s because Rails loads it for you as “models/foo” and you load it as “foo” so it gets loaded twice.

The moral of the story is: let Rails load the things it built, and you load the things you built.

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