Creating a Gem

A gem is a zip file containing code, typically, though not always Ruby code, plus a little metadata to help RubyGems and Bundler to manage it nicely. Gems contain a gemspec manifest file, which contains metadata, and usually a lib directory containing the code itself.

Because a gem contains Ruby code, it can do whatever you like. It can monkeypatch Rails. It can delare new modules and classes. It can extend existing objects, etc.

A gem can be stored locally, or on a gem host like If you publish your gem to rubygems it will be available for anyone to download and use. If you keep it in your project, you can still use bundler, you just provide a path.

In this section we're going to create a simple gem using Bundler.

Creating a gem using Bundler

We are going to create a summarise gem which will extend the string class with methods to create a summary, and to check whether the string can be summarised, like so:

class String
def summarise(l=200)
i = 0{ |word| word if (i += word.length) < l}.compact.join(' ')
def summarisable?(length=200)
return self.summarise(length) != self

first of all create the gem:

bundle gem summarise

We now have a new summarize directory containing a lib directory for code, a gemspec file for metadata, and a few other files.

We also have an empty git repository initialised for us.


The gemspec file is the heart of your gem, it contains all the metadata about your gem. Your generated gemspec will look something like this:

# coding: utf-8
lib = File.expand_path('../lib', __FILE__)
$LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
require 'summarise/version' do |spec| = "summarise"
spec.version = Summarise::VERSION
spec.authors = ["Nicholas Johnson"] = [""]
spec.description = %q{TODO: Write a gem description}
spec.summary = %q{TODO: Write a gem summary}
spec.homepage = ""
spec.license = "MIT"
spec.files = `git ls-files`.split($/)
spec.executables = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
spec.test_files = spec.files.grep(%r{^(test|spec|features)/})
spec.require_paths = ["lib"]
spec.add_development_dependency "bundler", "~> 1.3"
spec.add_development_dependency "rake"

Notice how spec.files is set by getting the files currently included in git.

Also notice how the version is taken from the Summarize::VERSION constant. This is defined in lib/summarise/version.rb. The version can be updated by editing this file.

Gem Contents

We now define the gem in the lib directory.


This file simply imports the rest of the code.

require "summarise/version"
require "summarise/string_extensions"
require "summarise/string"
module Summarise


This declares the methods we want to add.

module Summarise
module StringExtensions
def summarise(l=200)
i = 0{ |word| word if (i += word.length) < l}.compact.join(' ')
def summarisable?(length=200)
return self.summarise(length) != self


This extends the String class.

class String
include Summarise::StringExtensions

Building the gem

Because the gemspec uses Git to discover which files to include, we must first commit your updated files.

git add .
git commit -a -m "first commit"

Build the gem using the gem build command:

gem build summarise.gemspec

You will create a file called something like: summarise-0.0.1.gem

Local Private Gems

If you want to keep your gem private, you can deploy it directly into your vendor/gems directory, like so:

First unpack it into vendor/gems:

gem unpack summarise-0.0.1.gem --target /path_to_rails_app/vendor/gems/.

Now declare it in your Gemfile:

gem 'summarise', :path => "#{File.expand_path(__FILE__)}/../vendor/gems/summarise-0.0.1"

Finally install it into Gemfile.lock

bundle install

This is a good way to develop a gem, as you can deploy it locally and work on it in situ.

Uploading your gem to

If you'd like to share your gem with the community, you can also push it to

You'll need a RubyGems account:

Now simply push the packaged gem:

gem push summarise-0.0.1.gem

Exercise - creating a gem

Create a random string gem. I want to be able to call something like:


to get back a random alphanumeric string.

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