Adds collection_tag

This tag mirrors the post_tag functionality but for collections instead of just
posts.

lib/jekyll/tags/collection_url.rb

@@ -0,0 +1,47 @@
+module Jekyll
+  module Tags
+    class CollectionUrl < Liquid::Tag
+      TagName = 'collection_url'
+      MATCHER = /^([\w-]+)\s+\/?(.*)$/
+
+      def initialize(tag_name, markup, tokens)
+        super
+        orig_markup = markup.strip
+        all, @collection, @item_name = *orig_markup.match(MATCHER)
+        unless all
+          raise ArgumentError.new <<-eos
+Could not parse the collection or item "#{markup}" in tag '#{TagName}'.
+
+Valid syntax: #{TagName} <collection> <item>
+eos
+        end
+
+        @path_regex = /^\/#{@item_name}$/
+      end
+
+      def render(context)
+        site = context.registers[:site]
+
+        if site.collections[@collection]
+          site.collections[@collection].docs.each do |p|
+            return p.url if p.cleaned_relative_path.match(@path_regex)
+          end
+
+          raise ArgumentError.new <<-eos
+Could not find item "#{@item_name}" in collection "#{@collection}" in tag '#{TagName}'.
+
+Make sure the item exists and the name is correct.
+eos
+        else
+          raise ArgumentError.new <<-eos
+Could not find collection "#{@collection}" in tag '#{TagName}'
+
+Make sure the collection exists and the name is correct.
+eos
+        end
+      end
+    end
+  end
+end
+
+Liquid::Template.register_tag(Jekyll::Tags::CollectionUrl::TagName, Jekyll::Tags::CollectionUrl)

test/test_tags.rb

@@ -14,6 +14,10 @@ class TestTags < JekyllUnitTest
     if override['read_posts']
       site.posts.docs.concat(PostReader.new(site).read_posts(''))
     end
+    if override['read_collections']
+      # puts "reading collections"
+      CollectionReader.new(site).read
+    end
 
     info = { :filters => [Jekyll::Filters], :registers => { :site => site } }
     @converter = site.converters.find { |c| c.class == converter_class }
@@ -476,6 +480,73 @@ CONTENT
     end
   end
 
+  context "simple page with collection linking" do
+    setup do
+      content = <<CONTENT
+---
+title: Collection linking
+---
+
+{% collection_url methods yaml_with_dots %}
+CONTENT
+      create_post(content, {'source' => source_dir, 'destination' => dest_dir, 'collections' => { 'methods' => {}}, 'read_collections' => true})
+    end
+
+    should "not cause an error" do
+      refute_match /markdown\-html\-error/, @result
+    end
+
+    should "have the url to the \"yaml_with_dots\" item" do
+      assert_match %r{/methods/yaml_with_dots}, @result
+    end
+  end
+
+  context "simple page with nested collection linking" do
+    setup do
+      content = <<CONTENT
+---
+title: Collection linking
+---
+
+- 1 {% collection_url methods sanitized_path %}
+- 2 {% collection_url methods /sanitized_path %}
+- 3 {% collection_url methods site/generate %}
+- 4 {% collection_url methods /site/generate %}
+CONTENT
+      create_post(content, {'source' => source_dir, 'destination' => dest_dir, 'collections' => { 'methods' => {}}, 'read_collections' => true})
+    end
+
+    should "not cause an error" do
+      refute_match /markdown\-html\-error/, @result
+    end
+
+    should "have the url to the \"sanitized_path\" item" do
+      assert_match %r{1\s/methods/sanitized_path}, @result
+      assert_match %r{2\s/methods/sanitized_path}, @result
+    end
+
+    should "have the url to the \"site/generate\" item" do
+      assert_match %r{3\s/methods/site/generate}, @result
+      assert_match %r{4\s/methods/site/generate}, @result
+    end
+  end
+
+  context "simple page with invalid collection linking" do
+    should "cause an error" do
+      content = <<CONTENT
+---
+title: Invalid collection name linking
+---
+
+{% collection_url methods non-existent-collection-item %}
+CONTENT
+
+      assert_raises ArgumentError do
+        create_post(content, {'source' => source_dir, 'destination' => dest_dir, 'collections' => { 'methods' => {}}, 'read_collections' => true})
+      end
+    end
+  end
+
   context "include tag with parameters" do
 
     context "with symlink'd include" do