Ruby cheat sheet

A scannable Ruby reference: 7 short snippets across 5 topics, each linking back to the lesson it came from.

At a glance

Topic	What it covers
Standard library and a short note on Rails	Rails is a full framework built on Ruby: convention over configuration, MVC, and generators that write the boilerplate	lesson
Gems, Bundler and project layout	Use require_relative for files inside your own project and require for gems. Mixing them up makes a library work from	lesson
Testing with Minitest and RSpec	Minitest assertions, RSpec describe and it blocks, let and subject, doubles and mocks, and organising a suite that	lesson
Background jobs and the Ruby web stack	Jobs are arguments, not state. Pass identifiers and re-load records inside perform; serialising a large object graph	lesson
Deployment, performance and next steps	Bundler in production, Puma tuning, memory and GC behaviour, profiling with StackProf, structured logging, and where to	lesson

Quick snippets

Standard library and a short note on Rails

A short note on Rails

gem install rails
rails new blog
cd blog
bin/rails server

bin/rails generate model Post title:string body:text published_at:datetime
bin/rails generate controller Posts index show
bin/rails db:migrate

Full lesson: Standard library and a short note on Rails →

Gems, Bundler and project layout

Gemfile and the lockfile

bundle install                 # resolves and writes Gemfile.lock
bundle update rack             # move one gem within its constraint
bundle exec rspec              # run against locked versions
bundle outdated
bundle lock --add-platform x86_64-linux arm64-darwin

Full lesson: Gems, Bundler and project layout →

Testing with Minitest and RSpec

Doubles, mocks and stubs

# controlling time without a dependency
travel_to(Time.utc(2026, 6, 1)) do
  expect(subscription.renews_on).to eq(Date.new(2026, 7, 1))
end

# freezing randomness so a failure is reproducible
srand(1234)
expect(generator.pick).to eq("gamma")

Keeping the suite useful

bundle exec rspec --format progress --profile 10
bundle exec rspec spec/models/book_spec.rb:42   # run one example
bundle exec rake test                            # minitest through Rake
COVERAGE=1 bundle exec rspec                     # with simplecov enabled

Full lesson: Testing with Minitest and RSpec →

Background jobs and the Ruby web stack

Running the queue in production

bundle exec sidekiq -C config/sidekiq.yml -e production

# config/sidekiq.yml
# :concurrency: 10
# :queues:
#   - [critical, 4]
#   - [default, 2]
#   - [mail, 1]
# :max_retries: 5

Full lesson: Background jobs and the Ruby web stack →

Deployment, performance and next steps

Deploying Ruby

# deterministic install without touching the system gem path
bundle config set --local deployment true
bundle config set --local without "development test"
bundle config set --local path vendor/bundle
bundle install --jobs 4 --retry 3

# run behind a process manager
bundle exec puma -C config/puma.rb

# health check for the load balancer
curl -fsS http://127.0.0.1:3000/health || exit 1

Measuring before tuning

# structured logs a platform can index
require "json"
require "logger"

logger = Logger.new($stdout).tap { |l| l.formatter = proc do |severity, time, _prog, msg|
  JSON.generate(level: severity.downcase, ts: time.utc.iso8601(3), msg: msg) + "\n"
end }

logger.info("report built rows=#{rows} ms=#{elapsed_ms}")
# never log tokens, passwords, card data or full request bodies

Full lesson: Deployment, performance and next steps →

FAQ

Is this Ruby cheat sheet free to use?

Yes. No sign-up and no tracking: the page is static, every example is on the page itself, and you can print it or save it as a one-page reference.

Where do the examples come from?

Every snippet is taken from the 5 lessons of the Ruby course on this site, and each section links back to the lesson it was pulled from.

How do I go deeper than a cheat sheet?

Open the full Ruby course — it carries the worked explanations, the edge cases and the exercises behind every line here.

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Last refreshed 2026-09-27.