Adds easy defaults to writing Atom feeds with the Builder template engine (this does not work on ERB or any other template languages).

Full usage example:

config/routes.rb:
  Rails.application.routes.draw do
    resources :posts
    root to: "posts#index"
  end

app/controllers/posts_controller.rb:
  class PostsController < ApplicationController
    # GET /posts.html
    # GET /posts.atom
    def index
      @posts = Post.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[0].created_at) if @posts.length > 0

    @posts.each do |post|
      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:localhost,2005:/posts”, in this case.

• :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)

    @posts.each do |post|
      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.