rss: add per-tag Atom feed at /rss/tags/:tag/updates.xml

- rss#tag_updates action: filters Page.heads by tag name, default
  locale, 20 items, same caching as updates feed
- tag_updates.xml.builder: Atom feed with CGI.escapeHTML on title
  and summary, consistent with updates.xml.builder
- tags/show.html.erb: add subscription link above article list
- routes: two routes per existing pattern (format-less + .:format
  constrained to /xml/)

1 files changed, 18 insertions, 0 deletions

diff --git a/app/controllers/rss_controller.rb b/app/controllers/rss_controller.rb
index 4b47218..489a732 100644
--- a/app/controllers/rss_controller.rb
+++ b/app/controllers/rss_controller.rb
@@ -22,6 +22,24 @@ class RssController < ApplicationController
     end
   end
 
+  def tag_updates
+    expires_in 31.minutes, :public => true
+
+    I18n.locale = I18n.default_locale
+    @tag  = params[:tag]
+    @items = Page.heads
+      .joins("JOIN taggings ON taggings.taggable_id = pages.id
+          AND taggings.taggable_type = 'Page'
+          AND taggings.context = 'tags'")
+      .joins("JOIN tags ON tags.id = taggings.tag_id")
+      .where("LOWER(tags.name) = ?", @tag.downcase)
+      .order("published_at DESC").limit(20)
+
+    respond_to do |format|
+      format.xml {}
+    end
+  end
+
   def recent_changes
     @items = Page.where(
       "updated_at < ? AND updated_at > ?", Time.now, Time.now - 14.days