Script Belt: Detecting Funky Comparisons

Introduction

Welcome to another installment of “Script Belt,” where I share useful scripts that I regularly use for development or debugging. Today, we’re diving into a Ruby script that helps you detect funky comparisons in a Rails application. Probably this is very specific to my use case, but I have shared with some people that found some bits useful.

I wrote this script with the help of my friends at RubySur. I posted the first version of this script in our Slack channel and they helped me improve it.

Why Use This Script?

This one was born to help me check a reported bug was not happening in other places of the code. At some point we were comparing Date with Time and that was causing some issues. This script is helpful to find all the places where that was happening.

The Script

Here’s a part of the script that I included in my test_helper.rb. With this bit you might get the gist of it.

# test_helper.rb

# ...
# Custom logger for comparison operations
COMPARISON_LOGGER = Logger.new(STDOUT)
COMPARISON_LOGGER.level = Logger::WARN

class Date
  [:<, :<=, :==, :>, :>=, :===].each do |operator|
    define_method(operator) do |other|
      if other.class.in?([ActiveSupport::TimeWithZone, DateTime, Time])
        COMPARISON_LOGGER.warn "Comparison of Date with #{other.class} detected"
        caller_locations.each do |location|
          # Filter out traces from dependencies (gems)
          if location.absolute_path&.include?(Rails.root.to_s)
            COMPARISON_LOGGER.warn "#{location.absolute_path}:#{location.lineno}"
          end
        end
      end
      super(other)
    end
  end

  # patch methods that might not take just `other` as an arg

  [:between?, :clamp?].each do |operator|
    define_method(operator) do |*args|
      if other = args.detect { |arg| arg.class.in?([ActiveSupport::TimeWithZone, Range, DateTime, Time]) }

        COMPARISON_LOGGER.warn "Comparison of DateTime with #{other.class} detected"
        caller_locations.each do |location|
          # Filter out traces from dependencies (gems)
          if location.absolute_path&.include?(Rails.root.to_s)
            COMPARISON_LOGGER.warn "#{location.absolute_path}:#{location.lineno}"
          end
        end
      end
      super(*args)
    end
  end
end

How It Works

1. Custom Logger: The script uses a custom logger to print warnings when a comparison operation is detected between Date and Time classes.
2. Monkey Patching: It then monkey patches the comparison methods of the Date class to log a warning when a comparison operation is detected between Date and other classes.
3. Printing the Call Stack: If such a comparison operation is detected, the script prints the call stack to the console, filtering out traces from dependencies to make it easier to identify where the comparison is being made.

How to Use It

Simply add this script to your test_helper.rb, run your tests (rails test:all) and check if anything is logged to the console. If you see any warnings, you can use the printed call stack to identify where the comparison is being made.