How to run a Rails application within a gem? - ruby-on-rails

Question

I'm not sure if this sort of thing is very common, but I keep finding myself trying to create gems that are just wrappers around a Rails application.
My gem will have a generator to create a config.ru but the Rails application will live inside the gem's lib directory. I need to know how to "embed" a Rails application and configure it so that it can be run inside the gem.
For example:
$ mygem new project
mygem created a directory called "project" with the following files:
project/config.ru
project/widgets/
project/foobars/
My gem will also generate some directories that will need to be added to Rails somehow so that I can access the code in those directories from the Rails app living inside the Gem.
Any help or advice you can give me would be appreciated.
To clarify, I'm not trying to create a Rails engine, or plugin to a Rails application. I'm trying to create a fully-fledged Rails application, but package it as a gem so that a user of my gem can run the gem (the rails app) without needing to know that it's using Rails behind the scenes.
Update: Okay, I've got a little bit working now. I've created the gem and generated the rails project inside the gem's lib directory.
$ bundle gem my_gem && cd my_gem/lib
$ rails new my_gem --skip-bundle
Which leaves me with:
my_gem/
my_gem.gemspec
bin/my_gem
lib/
my_gem.rb
my_gem/
version.rb # generated by bundler
# the rails app:
app/
config/
Gemfile
...etc
Since this gem requires Rails, I started adding the gems defined in the Rails Gemfile as dependencies in the gem's Gemspec, but I'm a little confused as to how to handle the assets group in the Gemfile.
# Rails Gemfile
group :assets do
gem 'sass-rails', '~> 3.2.3'
gem 'coffee-rails', '~> 3.2.1'
gem 'therubyracer', :platforms => :ruby
gem 'uglifier', '>= 1.0.3'
end
# gemspec
Gem::Specification.new do |gem|
gem.name = "my_gem"
# ...
gem.add_dependency 'rails', '3.2.8'
gem.add_dependency 'sqlite3'
gem.add_dependency 'jquery-rails'
# how to add the assets group gems?
end

Answer 1

Try this and see if it helps you make progress.
Gems are just directories of files, and you can put whatever files you want into a gem.
Create:
Create a blank gem full-blown Rails project:
$ bundle gem my_gem
Then a Rails app:
$ rails new my_app --skip-bundle
Copy the Rails files into the gem:
$ cp -R my_app/* my_gem
Bundle everything into your Rails app:
$ cd my_gem
$ bundle install --binstubs --path vendor/bundle
$ cd -
Make the Rakefile have the gem tasks and the Rails setup:
#!/usr/bin/env rake
require "bundler/gem_tasks"
require File.expand_path('../config/application', __FILE__)
MyApp::Application.load_tasks
Verify that it starts:
$ rails server
Load Path:
To control where Rails looks for files, such as "external" configuration files, you can use the file config/application.rb with any directory paths like this:
# Add additional load paths for your own custom dirs
# config.load_paths += %W( #{config.root}/../customdir )
Note the ".." which means go above the Rails directory. This gives you a path relative to the gem.
If you prefer you can specify an absolute path, for example if you know the user will always keep his external files in "~/myfiles/". You can also choose to use ENV vars to send in whatever directory you want.
If you read about load path capabilties, look for lines that are shorthand for adding a directory to the front of the load path because you may want to put your external diretories first:
$:.unshift File.dirname(__FILE__)
Gem Build:
Edit my_gem.gemspec to add your own description, homepage, summary, etc. then build:
$ gem build my_gem.gemspec
Successfully built RubyGem
Name: my_gem
Version: 0.0.1
File: my_gem-0.0.1.gem
Now your Rails app is packaged as a gem.
The config.ru should be a typical Rails one. No special changes AFAIK.
When your user wants to install your app:
$ gem install my_gem
The gem will install in the user's typical gem directory. If you want to adjust this, see this page on rubygems: http://docs.rubygems.org/read/chapter/3
Crate:
You may also want to investigate the Crate project:
Crate: Packaging Standalone Ruby Applications
http://www.slideshare.net/copiousfreetime/crate-packaging-standalone-ruby-applications
Rack:
To use config.ru here is the typical Rails setup:
# Rails.root/config.ru
require "config/environment"
use Rails::Rack::LogTailer
use ActionDispatch::Static
run ActionController::Dispatcher.new
For your project, you want to require some files before Rails. You'll want to learn about the Ruby "require" and how it finds files using LOAD_PATH.
The easy way:
# Rails.root/config.ru
require_relative 'filename'
require "config/environment"
Or to put the user's custom directory up couple directory levels:
require './../../filename' # not the best for security
Or to use an absolute path, read about File.expand_path:
File.expand_path(__FILE__)
Or to use the current directory and put it on the load path:
$LOAD_PATH.unshift(File.dirname(__FILE__))
require 'filename'
Lots of choices for you to consider. Hope this helps!

Answer 2

What about the question, "How am I going to run the Rails application inside the gem?".
A Rails application has controllers and views to run a web server. What you need are actions to create, list, update, and destroy. Exposing these actions without a web server is essentially having such methods in a class. That's a normal standard type of gem in the first place.
So maybe your questions is really, how do I write a gem where I have ActiveRecord, and the other Rails stuff.
First, you need to make your gem dependent on the Rails gems you need. You do this in the gemspec file for your gem.
Then it really is just a matter of your gem code doing a require of the right Rails gems you need.

Answer 3

I'm not sure if this will help, as I read through everything and I couldn't find the motivation behind why you were doing this. One of the reasons I came up with was making something that can be used on a desktop environment. In that case you could try using something like Bowline. If you just want to provide an application that others can download and use and install themselves, then you can probably assume they can follow at least basic developer kind of instructions and you could just provide the whole app on github or as a zip file. See an example of someone else doing something similar over on Fat Free CRM's github page.