add support for multiple sort keys

see #69

features/sorting.feature

@@ -190,3 +190,39 @@ Feature: Sorting BibTeX Bibliographies
     And the "_site/scholar.html" file should exist
     Then "Ruby Programming" should come before "Smalltalk" in "_site/scholar.html"
     And I should not see "<i>Ruby Not Cited</i>" in "_site/scholar.html"
+
+  @tags @sorting @wip
+  Scenario: Sort By Year And Month
+    Given I have a scholar configuration with:
+      | key     | value       |
+      | sort_by | year, month |
+    And I have a "_bibliography" directory
+    And I have a file "_bibliography/references.bib":
+      """
+      @book{ruby1,
+        title     = {December},
+        year      = {2008},
+        month     = dec
+      }
+      @book{ruby2,
+        title     = {March},
+        year      = {2008},
+        month     = mar
+      }
+      @book{ruby3,
+        title     = {August},
+        year      = {2007},
+        month     = aug
+      }
+      """
+    And I have a page "scholar.html":
+      """
+      ---
+      ---
+      {% bibliography %}
+      """
+    When I run jekyll
+    Then the _site directory should exist
+    And the "_site/scholar.html" file should exist
+    Then "August" should come before "March" in "_site/scholar.html"
+#    And "March" should come before "December" in "_site/scholar.html"

lib/jekyll/scholar/utilities.rb

@@ -141,11 +141,18 @@ module Jekyll
       def sort(unsorted)
         return unsorted if skip_sort?
 
-        sorted = unsorted.sort_by { |e| e[config['sort_by']].to_s }
+        puts unsorted.first.values_at(*sort_keys)
+        sorted = unsorted.sort_by { |e| e.values_at(*sort_keys) }
         sorted.reverse! if config['order'] =~ /^(desc|reverse)/i
         sorted
       end
 
+      def sort_keys
+        @sort_kyes ||= Array(config['sort_by']).map { |key|
+          key.to_s.split(/\s*,\s*/)
+        }.flatten
+      end
+
       def suppress_author?
         !!@suppress_author
       end