Full usage example:

  config/routes.rb:
    ActionController::Routing::Routes.draw do |map|
      map.resources :posts
      map.root :controller => "posts"
    end

  app/controllers/posts_controller.rb:
    class PostsController < ApplicationController::Base
      # GET /posts.html
      # GET /posts.atom
      def index
        @posts = Post.find(:all)

        respond_to do |format|
          format.html
          format.atom
        end
      end
    end

  app/views/posts/index.atom.builder:
    atom_feed do |feed|
      feed.title("My great blog!")
      feed.updated(@posts.first.created_at)

      for post in @posts
        feed.entry(post) do |entry|
          entry.title(post.title)
          entry.content(post.body, :type => 'html')

          entry.author do |author|
            author.name("DHH")
          end
        end
      end
    end

The options for atom_feed are:

• :language: Defaults to "en-US".
• :root_url: The HTML alternative that this feed is doubling for. Defaults to / on the current host.
• :url: The URL for this feed. Defaults to the current URL.
• :id: The id for this feed. Defaults to "tag:#{request.host},#{options[:schema_date]}:#{request.request_uri.split(".")}"
• :schema_date: The date at which the tag scheme for the feed was first used. A good default is the year you created the feed. See feedvalidator.org/docs/error/InvalidTAG.html for more information. If not specified, 2005 is used (as an "I don‘t care" value).
• :instruct: Hash of XML processing instructions in the form {target => {attribute => value, }} or {target => [{attribute => value, }, ]}

Other namespaces can be added to the root element:

  app/views/posts/index.atom.builder:
    atom_feed({'xmlns:app' => 'http://www.w3.org/2007/app',
        'xmlns:openSearch' => 'http://a9.com/-/spec/opensearch/1.1/'}) do |feed|
      feed.title("My great blog!")
      feed.updated((@posts.first.created_at))
      feed.tag!(openSearch:totalResults, 10)

      for post in @posts
        feed.entry(post) do |entry|
          entry.title(post.title)
          entry.content(post.body, :type => 'html')
          entry.tag!('app:edited', Time.now)

          entry.author do |author|
            author.name("DHH")
          end
        end
      end
    end

The Atom spec defines five elements (content rights title subtitle summary) which may directly contain xhtml content if :type => ‘xhtml’ is specified as an attribute. If so, this helper will take care of the enclosing div and xhtml namespace declaration. Example usage:

   entry.summary :type => 'xhtml' do |xhtml|
     xhtml.p pluralize(order.line_items.count, "line item")
     xhtml.p "Shipped to #{order.address}"
     xhtml.p "Paid by #{order.pay_type}"
   end

atom_feed yields an AtomFeedBuilder instance. Nested elements yield an AtomBuilder instance.