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/)

4 files changed, 54 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
diff --git a/app/views/rss/tag_updates.xml.builder b/app/views/rss/tag_updates.xml.builder
new file mode 100644
index 0000000..6491cfc
--- /dev/null
+++ b/app/views/rss/tag_updates.xml.builder
@@ -0,0 +1,27 @@
+xml.instruct!
+
+xml.feed(:xmlns => "http://www.w3.org/2005/Atom", "xml:base" => @host) do
+  xml.title("Chaos Computer Club: #{@tag}")
+  xml.link(:href => "#{@host}/")
+  xml.link(:rel => "self", :href => "#{@host}/rss/tags/#{@tag}/updates.xml")
+  xml.updated(@items.first.published_at.xmlschema) unless @items.empty?
+  xml.author do
+    xml.name("Chaos Computer Club e. V.")
+  end
+  xml.id("#{@host}/rss/tags/#{@tag}/updates")
+
+  @items.each do |item|
+    xml.entry do
+      xml.title(CGI.escapeHTML(item.title.to_s))
+      xml.link(
+        :href => content_url(:page_path => item.node.unique_path),
+        :rel  => "alternate",
+        :type => "text/html"
+      )
+      xml.id(content_url(:page_path => item.node.feed_id))
+      xml.updated(item.updated_at.xmlschema)
+      xml.published(item.published_at.xmlschema)
+      xml.summary(CGI.escapeHTML(item.abstract.to_s))
+    end
+  end
+end
diff --git a/app/views/tags/show.html.erb b/app/views/tags/show.html.erb
index b09cc15..3a23522 100644
--- a/app/views/tags/show.html.erb
+++ b/app/views/tags/show.html.erb
@@ -1,5 +1,11 @@
 <h2><%= t(:show_tag_headline) %> »<%=h @tag %>«</h2>
 
+<p class="feed_link">
+  <%= link_to "Subscribe to updates tagged »#{@tag}«",
+        rss_tag_url(:tag => @tag, :format => :xml),
+        :type => "application/atom+xml" %>
+</p>
+
 <%= 
     render(
       :partial => 'custom/partials/no_date_and_author',
diff --git a/config/routes.rb b/config/routes.rb
index 2df9d46..92f2452 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -70,6 +70,9 @@ Cccms::Application.routes.draw do
     get  'rss/updates.:format', :to => 'rss#updates', :as => :rss_feed,
            :constraints => { :format => /xml|rdf/ }
     get  'rss/recent_changes',  :to => 'rss#recent_changes'
+    get  'rss/tags/:tag/updates',         :to => 'rss#tag_updates', :as => :rss_tag
+    get  'rss/tags/:tag/updates.:format', :to => 'rss#tag_updates', :as => :rss_tag_feed,
+           :constraints => { :format => /xml/ }
 
     match 'galleries/*page_path' => 'content#render_gallery', :via => :get
     match '/*page_path'          => 'content#render_page', :as => :content, :via => :get