Creating a gem in lib folder

Go to lib folder and use bundler to generate main files for a gem:

$ bundle gem test_gem

      create  test_gem/Gemfile
      create  test_gem/Rakefile
      create  test_gem/LICENSE
      create  test_gem/README.md
      create  test_gem/.gitignore
      create  test_gem/test_gem.gemspec
      create  test_gem/lib/test_gem.rb
      create  test_gem/lib/test_gem/version.rb
Initializating git repo in /path/to/webapp/HouseTrip-Web-App/lib/test_gem

cd in to created directory

$ cd test_gem/

Bundler automatically created git repository for us, but we do not need it. Lets remove it:

$ rm -rf .git/

And we need to remove git dependency from gemspec file, open test_gem.gemspec file and change this line :

gem.files         = `git ls-files`.split($\)

to this :

gem.files         = Dir.glob("{bin,lib}/**/*")

Next step is to configure rake to include gem files properly and to include all rake files from a gem.
Open Rakefile file and overwrite it with this code:

#!/usr/bin/env rake
require "bundler/gem_tasks"

$LOAD_PATH << File.join(File.dirname(__FILE__), 'lib')
$LOAD_PATH << File.join(File.dirname(__FILE__))

# load custom tasks
Dir.glob(["lib/**/*.rake", "tasks/**/*.rake"]).each do |file|
  Kernel.load file
end

And the last step is to add our gem in to project, open Gemfile from main web app and add this line:

gem 'test_gem', :path => File.join(File.dirname(__FILE__), '/lib/test_gem')

DONE :)