Render Options in Rails 3


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In Inside Rails, Yehuda Katz, Rails expert and core team member, and Carl Lerche, Rails expert and full-time contributor, present expert advice and insight on the Rails platform and Rails development.

In previous versions of Rails, adding a new rendering option to Rails required performing an alias_method_chain on the render method, adding your new options, and hoping they didn’t conflict with any of the other code in the rendering pipeline.

Rails 3 makes rendering options a first class citizen, and uses the same new system internally that plugins authors are expected to use. Before we get into how you can use this feature yourself, let’s take a look at how Rails uses it:

ActionController.add_renderer :json do |json, options|
json = ActiveSupport::JSON.encode(json) unless json.respond_to?(:to_str)
json = "#{options[:callback]}(#{json})" unless options[:callback].blank?
self.content_type ||= Mime::JSON
self.response_body = json

Here, we are creating a new render :json option, which behaves exactly like render :json in Rails 2.3. In the render_json method, we use the same lower-level (but still public) APIs that are used by Rails itself to set the MIME type and response body. Using a render option entirely skips the rest of the render pipeline, so you don’t have to worry about blocking normal template selection and rendering, or any other internal changes that might break your added option.

Next, let’s take a look at adding a new render :pdf option. We’ll use JRuby and the Flying Saucer library, which takes HTML and CSS and converts it to a PDF file. The syntax for the new option will be render :pdf => "template_name", :css => %w(main.css print.pdf), :layout => "print".

In order to understand how to do this, let’s first take a look at the code necessary to use Flying Saucer in JRuby. First, you’ll need to grab the Flying Saucer jars (you can get them from my Muse git repo). Next, let’s write the code necessary to convert HTML and CSS to PDF with Flying Saucer:

# Both of these are .jar files
require "/path/to/itext"
require "/path/to/core-renderer"

module PdfUtils
def self.string_to_pdf(input_string)
io =
renderer =
renderer.set_document_from_string_input input_string
io.string # the PDF file in a String

It’s a bit verbose, but it does work. Now that we have a way to make the PDF, let’s wire it up into a render option.

ActionController.add_renderer :pdf do |template, options|
css_files = Array.wrap(options.delete(:css))
css = {|file| }.join("\n")

# Reuse the render semantics to get a string from the
# template and options
string = render_to_string template, options

# Drop in the CSS before the in a style tag.
# In practice, you would probably cache the file reads
# and the merging of the CSS and HTML.
string.gsub!(%r{}, "

send_data PdfUtils.string_to_pdf(string), :type => Mime::PDF

You’d also need to register the PDF mime type:

Mime::Type.register "application/pdf", :pdf

You could now do the following in a controller:

class PostsController &lt; ActionController::Base
def show
@post = Post.find(params[:id])

respond_to do |format|
# or
format.pdf { render :pdf => "show", :css => %w(application print) }

The tricky bits of this process are now reserved to figuring out how to build and return the output, not how to inject your option into render. Pretty cool, no?

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