Using rsruby in RubyOnRails

1. Create your Rails application
Let’s call it Plotter. Usual procedure:


rails Plotter
cd Plotter
rake db:create

We won’t use the database (default sqlite3) in this example, but you may use it later on.

2. Tell the application about R_HOME
RSRuby will fail unless the environment variable R_HOME is set.
To tell Plotter that R_HOME is /usr/lib/R, I put this in the file app/helpers/application_helper.rb:


module ApplicationHelper
# set R_HOME if not set
if ENV['R_HOME'].nil?
ENV['R_HOME'] = "/usr/lib/R"
end
end

3. Make an instance of RSRuby available to all controllers
RSRuby works by providing an instance of an “R object”, on which you call R functions as methods.
I provided the object by editing app/controllers/application_controller.rb to include a method named InitR:


class ApplicationController < r =" RSRuby.instance">
Create a sample controller and view
For testing purposes, I created a controller named Home with a single view named index:

./script/generate controller Home index
rm public/index.html
I edited config/routes.rb so as the root of the application is the index view for the home controller:

ActionController::Routing::Routes.draw do |map|
map.root      :controller => "home"  #default view is app/views/home/index.html.erb
map.connect  ':controller/:action/:id'
map.connect  ':controller/:action/:id.:format'
end
Now we can get to work on the file app/controllers/home_controller.rb.  I wrote a sample method, show_image to plot a histogram using R via RSRuby:

  def show_image
# next 6 lines use R to plot a histogram
@r = InitR()
@d = @r.rnorm(1000)
@l = @r.range(-4,4,@d)
@r.png "/tmp/plot.png"
@r.par(:bg => "cornsilk")
@r.hist(@d, :range => @l, :col => "lavender", :main => "My Plot")
@r.eval_R("dev.off()")  #required for png output
# then read the png file and deliver it to the browser
@g = File.open("/tmp/plot.png", "rb") {|@f| @f.read}
send_data @g, :type=>"image/png", :disposition=>'inline'
end
Not the prettiest code, but you get the idea. Obviously the owner of the HTTP daemon (www-data on Ubuntu) must have write permission to the location of the PNG file; /tmp in this case.
Create the view
All that remains is to edit app/views/home/index.html.erb so as it displays the image:


Home#index
Find me in app/views/home/index.html.erb


<%= image_tag(url_for(:controller => "home", :action => "show_image")) %>
Which just says “wrap whatever comes out of the method show_image in an IMG SRC tag”.