Ruby/Rails console tricks I always forget

A compilation of tricks I use in irb and rails console. Most are one-liners to investigate bugs, measure performance, or just make the terminal more pleasant.

List IRB history

puts Readline::HISTORY.to_a

Useful when you know you typed something correct in a previous session but can’t remember exactly what it was.

Use link_to directly in the console

include ActionView::Helpers::UrlHelper

After that, link_to "x", "/y" works in rails console.

Inspect a route as the app sees it

app.users_path

The app object in rails console exposes route helpers as if you were inside a request.

See ActiveRecord SQL in the console

ActiveRecord::Base.logger = Logger.new STDOUT

From then on, every User.where(...) prints the generated SQL. Good for understanding N+1 queries and indexes on the spot.

List all environment variables

ENV

Yes, that’s all. But it’s exactly what you forget in production.

Save an object as JSON to debug at your own pace

File.write('public/debug_object.json', offers.to_json)

Then I open it in my editor with formatting to understand large structures.

Measure the time of a single line

puts Benchmark.measure {
  y = User.all.pluck(:id);
}

Benchmark.measure returns user/system/total/real time. A good first approximation before reaching for a profiler.

Hunt memory leaks with memory_profiler

require 'memory_profiler'

MemoryProfiler.start
# ... run what you want to measure here ...
report = MemoryProfiler.stop
report.pretty_print(scale_bytes: true, to_file: 'log/memory_profile.txt')

Generates a report with allocation hotspots. Ideal for debugging jobs that keep growing in RAM.

Colors in IRB without any gem

def red(str)
  "\033[31m#{str}\033[0m"
end

def green(str)
  "\033[32m#{str}\033[0m"
end

def blue(str)
  "\033[34m#{str}\033[0m"
end

def yellow(str)
  "\033[33m#{str}\033[0m"
end

Paste into ~/.irbrc and use puts green("ok") to highlight output in long scripts.

Uninstall all gems from the machine

for x in `gem list --no-versions`; do gem uninstall $x -a -x -I; done

Useful when the gem system turns into a mess and you want to start from scratch. Be careful on machines with a system Ruby — prefer running this inside an rbenv/asdf environment.

Where to find gems for a new problem

ruby-toolbox.com/categories — has adoption and activity rankings by category, helps avoid abandoned gems.