I recently had one of those moments where some code failed, but not in the way I expected it to. To create a contrived example, the scenario was basically this:

class LocationsController < ApplicationController
  def update
    my_location = Location.find(params[:id])
    do_something_with(location)
  end
end

This code won’t do what I want it to because I have defined the variable my_location but then passed location to another method. What I initially expected upon finding the bug is that I would have seen a NameError complaining that the local variable location didn’t exist. But I didn’t. As far as I could tell, location existed and it was nil.

It wasn’t referenced anywhere else in LocationsController, nor in ApplicationController. So what is it? Fortunately, Ruby provides a great way to find out.

class LocationsController < ApplicationController
  def update
    puts method(:location).source_location
    # "/usr/local/bundle/gems/actionpack-5.0.7.2/lib/action_controller/metal.rb"
    # 149
  end
end

That’s that mystery solved. There is a method called location implemented a layer or two up the inheritance hierarchy. We knew it had to come from somewhere, but source_location lets us determine the exact location.