Project Management

Ruby Course


You may have been wondering how programmers structure their projects. Do they just put their entire code in a single file? If they don’t, how can they use code across multiple files? Are there agreed-upon rules? Common patterns? What is the Ruby way of doing things?

In this lesson, we will show you how to structure your Ruby projects to keep them manageable and to help you and others easily navigate the code you’ve written.

Lesson overview

This section contains a general overview of topics that you will learn in this lesson.

  • The benefits of splitting your code into separate files.
  • How to make use of code from a separate file.
  • Namespace and scope of required files.
  • What development dependencies are.
  • What gems are.
  • What Bundler is and why it is useful.
  • What bundle init does and what a Gemfile is.
  • How to get the most out of Ruby LSP VSCode extension.

Confusion, convention, convenience

Recall how the projects you worked on in Foundations were structured. The HTML, CSS and JS all lived in separate files and appeared to be “one” in the browser thanks to linking CSS and JS in the HTML file(s).

Organizing your projects into different files has many practical benefits. Chief among them is making your code more modular, making it easier to adjust and understand the code as it gets more complex.

Remember that old saying about how to keep your physical environment organized - “A place for everything and everything in its place”. The same principle applies to software projects.

For Ruby projects, the rule of thumbs are:

  • One class per file. Every time you create a new class, you should create a new file for it to live in.
  • It is convention to put all your Ruby files into a lib directory. For example:
├── lib
│   └── lovely_file_of_yours.rb
└── main.rb

Making use of multiple files

If you are to split your code across multiple files, you first will need to know how to make sure code from one file can be used in another file. Let’s consider this file structure:

├── lib
│   ├── sort
│   │   ├── bogo_sort.rb
│   │   ├── bubble_sort.rb
│   │   └── merge_sort.rb
│   └── sort.rb
└── main.rb

There are two main ways to do that: require_relative and require.


# You're in the root of the project, the directory that holds main.rb

# main.rb
require_relative 'lib/sort'

# sort.rb
require_relative 'sort/bubble_sort'
require_relative 'sort/bogo_sort'
require_relative 'sort/merge_sort'

Let’s start with how the docs define its functionality:

require_relative(string) → true or false Ruby tries to load the library named string relative to the directory containing the requiring file. If the file does not exist a LoadError is raised. Returns true if the file was loaded and false if the file was already loaded before.

The important part here is relative to the directory containing the requiring file. This means that no matter where you execute the code from, require_relative looks for the file specified from the point of view of the file it has been written in. So main.rb is simply going to lib to find sort (the .rb is implicit), and sort.rb is going to sort to find those three different sorts. Simple enough, isn’t it?


require is trickier. Let’s grab some of the docs here:

If the feature is an absolute path (e.g. starts with '/'), the feature will be loaded directly using the absolute path. If the feature is an explicit relative path (e.g. starts with './' or '../'), the feature will be loaded using the relative path from the current directory. Otherwise, the feature will be searched for in the library directories listed in the $LOAD_PATH.

The absolute path bit seems self-explanatory. When you use a relative path the difference between using a relative path with require and doing require_relative is that require’s relative paths are resolved from the point of view of the directory you are running your code from. Let’s change our example:

# You're in the root of the project, the directory that holds main.rb

# main.rb
require 'lib/sort'

# sort.rb
require 'sort/bubble_sort'
require 'sort/bogo_sort'
require 'sort/merge_sort'

Ah. Of course - an error - it can’t find lib/sort! Those are not relative paths… Fancy schmancy require_relative and its implicitly assuming the paths are relative!

# You're in the root of the project, the directory that holds main.rb

# main.rb
require './lib/sort'

# sort.rb
require './sort/bubble_sort'
require './sort/bogo_sort'
require './sort/merge_sort'

Now it says it can’t find ./sort/bubble_sort! This is because it is not looking for it from the point of view of sort.rb but from the point of view of main.rb.

What about the $LOAD_PATH part?

# You're in the root of the project, the directory that holds main.rb

# main.rb
require 'csv'

require_relative 'lib/sort'

require 'csv' is going to look for a csv.rb in the Ruby’s $LOAD_PATH global variable which by default contains the Ruby standard library. There are other file extensions it might look for, but this is not important at this point - just remember that the requires look for some extensions like .rb without the need to declare them explicitly. In addition to that, if it doesn’t find that file in $LOAD_PATH it is going to look through installed gems (more on those later) to see if the file is there.

Both of those approaches (require and require_relative) are going to execute the file, allowing you to use their contents. If you try to require something for the second time, nothing will happen, and the requires will return false.

Convention is that require_relative is used for your own code, while require is used for things outside of it, like gems that your app depend on.

Benefit of this approach is that you don’t need to hold all the code for part of your app in one file:

# You're in the root of the project, the directory that holds main.rb

# This is your file structure:
├── lib
│    ├── flight.rb
│    ├── hotel.rb
│    └── airport.rb
└── main.rb

# lib/airport.rb
class Airport
  def introduce
    puts "I'm at the airport!"

# lib/flight.rb
class Flight
  def introduce
     puts "I'm on the flight!"

# lib/hotel.rb
class Hotel
  def introduce
     puts "I'm at the hotel!"

# main.rb
require_relative 'lib/airport'
require_relative 'lib/flight'
require_relative 'lib/hotel'
#=> I'm at the airport!
#=> I'm on the flight!
#=> I'm at the hotel!

So instead of defining both the Flight and Hotel classes inside airport.rb, we can do that in separate files. It is customary to require all the files in your topmost file, like main.rb here. This allows everyone to just get hold of main.rb and they get the entirety of your code where they need it. Depending on their needs, they would use an appropriate way of loading that file.

Another thing to keep in mind is that local variables do not get loaded, so if your airport.rb had a local variable coolest_airports, trying to access it in main.rb would raise an error. Constants do get loaded however, so you can access those.

Something important to keep in mind is that all required code is put into the same namespace. This means that if you have the same names for methods, modules, classes and so on they will be added together in order they were required. For example, let’s say you and your friend have used the same method name and you’re trying to use their code and yours:

# all files are in the same directory for simplicity's sake

# not_so_green.rb
def food_opinion(food)
  `#{food} is awesome!`

# scheals.rb
def food_opinion(food)
  `#{food} is awful!`

# main.rb
require_relative 'not_so_green'
require_relative 'scheals'

puts food_opinion('Cereal')
#=> Cereal is awful!
# Since food_opinion is defined twice, the last definition wins out.

To make sure code doesn’t get overwritten, Rubyists wrap their code in modules which give them the benefits of a namespace:

# all files are in the same directory for simplicity's sake

# not_so_green.rb
module NotSoGreen
  def self.food_opinion(food)
    `#{food} is awesome!`
# scheals.rb
module Scheals
  def self.food_opinion(food)
    `#{food} is awful!`
# main.rb
require_relative 'not_so_green'
require_relative 'scheals'

puts NotSoGreen.food_opinion('Cereal')
#=> Cereal is awesome!
puts Scheals.food_opinion('Marmite')
#=> Marmite is awful!
puts food_opinion('Cereal')
#=> Errors out - there's no longer a free floating foo method to use.

Gems and you

Now that you know how to work with your own files, it is time to learn how to work with the files of others.

Gems are packages containing Ruby utility libraries that someone wrote - basically, some code. Some of those gems are part of the Ruby standard library, but most require installing independently.

If you use a gem then you call such a gem a dependency - your code depends on that gem to function properly. Some dependencies are only used in particular contexts; for example, you can have a set of gems used only in a development or test environment.

Many gems depend upon other gems, and sometimes, the versions they depend on differ. You could manage gems and their dependencies on your own, going through the process of installing, updating them and resolving whatever conflicts might arise, but Rubyists have tools for that!

RubyGems has been part of Ruby standard library since version 1.9 and is used to get those amazing gems onto your computer. Remember that bit about require going through installed gems to potentially find a file you’re looking for? That’s the work of RubyGems. Another cool part about it? It is a gem itself! Let’s give it a try.

Create a new Ruby file main.rb in a directory called colorful:

require 'colorize'

puts 'Red goes faster!'.colorize(:red)

puts "I'm blue da ba dee da ba di!".colorize(:blue)

puts "It ain't easy bein' green...".colorize(:green)

You’re probably itching to see all those colours, so run your file with ruby main.rb to see them… or rather, a LoadError. Right - you need to install that gem first! Do that with gem install colorize and you’ll see RubyGems in action. Your system now has access to the Colorize gem!

Wait, your system - what about others who would like to use your code? Yeah, they would also need to gem install it - no big deal.

But what if you have dozens of gems? How do you ensure that the versions you use are the same version others download? This sounds rather tedious. Enter: Bundler. It’s another gem, part of RubyGems, but released independently.

Bundler allows you to declare what gems your project needs - down to their version. As for others, Bundler allows them to take that declaration, a simple file called Gemfile, and use it to install those gems in a quick bundle install.

Since gem installs are global, you need a way to run only those particular gem versions that are declared in the Gemfile. You can do that by using bundle exec followed by a command you want to execute - most likely bundle exec ruby foo.rb.

Let’s make sure whoever wants to use our script can do so:

bundle init # creates a default Gemfile in the current working directory
bundle add colorize # adds the colorize gem to the Gemfile and runs bundle install

This has created two files: Gemfile and Gemfile.lock. Let’s take a look into both:

# Gemfile

# frozen_string_literal: true

source ""

# gem "rails"

gem "colorize", "~> 1.1"

# Gemfile.lock
    colorize (1.1.0)

  x86_64-linux # This might be different for you if you're using a different CPU and OS.

  colorize (~> 1.1)


There’s not much in those, but as you can see, the Gemfile has information on where to get the gems from and what gems are required.

The "~> 1.1" is a version constraint, particularly a pessimistic constraint. It relies on semantic versioning.

  • The first number is the major version
  • The second is the minor version
  • the third, if it exists, is the patch number

Major versions can break things from previous versions - for example, changing method names. Minor versions can add and change things but can’t break anything. Patches happen when you introduce bug fixes that don’t break anything.

So, if people behind a gem maintain it in line with semantic versioning, you can rely on this pessimistic constraint never letting your project have a gem version that could potentially break your app - it is equivalent to gem "colorize", ">= 1.1", "<2.0".

Gemfile.lock has information on what was the last environment that should be able to run your app. Bundler will use it to install the same versions of gems even if Gemfile could potentially allow for newer versions to be installed.


There’s another important thing to give to folks that will run your code: the target Ruby version of your project. You can do it easily by running rbenv local 3.2.2 as it creates a .ruby-version file with the version declared.

Many other tools recognize this to figure out what Ruby version your project is running - for example, rbenv will no longer use the global Ruby version and the Ruby LSP VSCode extension will also change its behavior.

Ruby LSP in VSCode

Earlier in the curriculum, you were instructed to choose the Don't show again option when Ruby LSP told you about not finding a lock file - you might’ve also seen errors concerning RuboCop. In the next lesson, we will review RuboCop and how it should land in your project’s Gemfile.

After that, you will enjoy all the benefits of using Ruby LSP and its RuboCop integration, and your projects will be set up like a real pro.


  1. Read Eric Mathison’s Understanding require and Friends in Ruby.
  2. Check out the RubyGems Basics.
  3. Skim through Patterns guide from RubyGems, paying closer attention to Semantic versioning, Declaring dependencies and Loading code.
  4. Skim through Bundler’s Getting Started and Frequently Asked Questions.
  5. Check out Ruby LSP VSCode extension’s README.

Knowledge check

The following questions are an opportunity to reflect on key topics in this lesson. If you can’t answer a question, click on it to review the material, but keep in mind you are not expected to memorize or master this knowledge.

Additional resources

This section contains helpful links to related content. It isn’t required, so consider it supplemental.

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