add interpolation support to query parameter

features/bibtex.feature

@@ -151,6 +151,41 @@ Feature: BibTeX
     And I should not see "<i>The Ruby Programming Language</i>" in "_site/scholar.html"
     And I should see "<i>Smalltalk Best Practice Patterns</i>" in "_site/scholar.html"
 
+  @tags @filter @variables @wip
+  Scenario: Filter using interpolated query variable
+    Given I have a scholar configuration with:
+      | key    | value             |
+      | source | ./_bibliography   |
+    And I have a "_bibliography" directory
+    And I have a file "_bibliography/references.bib":
+      """
+      @book{ruby,
+        title     = {The Ruby Programming Language},
+        author    = {Flanagan, David and Matsumoto, Yukihiro},
+        year      = {2008},
+        publisher = {O'Reilly Media}
+      },
+      @book{smalltalk,
+        title     = {Smalltalk Best Practice Patterns},
+        author    = {Kent Beck},
+        year      = {1996},
+        publisher = {Prentice Hall}
+      }
+
+      """
+    And I have a page "scholar.html":
+      """
+      ---
+      ---
+      {% assign yr = 2000 %}
+      {% bibliography -f references --query @book[year <= {{ yr }}] %}
+      """
+    When I run jekyll
+    Then the _site directory should exist
+    And the "_site/scholar.html" file should exist
+    And I should not see "<i>The Ruby Programming Language</i>" in "_site/scholar.html"
+    And I should see "<i>Smalltalk Best Practice Patterns</i>" in "_site/scholar.html"
+
   @tags @bibliography @prefix
   Scenario: A Prefixed Bibliography
     Given I have a scholar configuration with:

lib/jekyll/scholar/utilities.rb

@@ -14,8 +14,7 @@ module Jekyll
     # #site readers
     module Utilities
 
-      attr_reader :config, :site, :query,
-        :context, :prefix, :text, :max
+      attr_reader :config, :site, :context, :prefix, :text, :max
 
       def split_arguments(arguments)
 
@@ -123,6 +122,10 @@ module Jekyll
         @bibliography
       end
 
+      def query
+        interpolate @query
+      end
+
       def entries
         sort bibliography[query || config['query']]
       end
@@ -387,6 +390,14 @@ module Jekyll
         end
       end
 
+      def interpolate(string)
+        return unless string
+
+        string.gsub(/{{\s*([\w\.]+)\s*}}/) do |match|
+          context.send(:resolve, $1) || match
+        end
+      end
+
       def set_context_to(context)
         @context, @site, = context, context.registers[:site]
         config.merge!(site.config['scholar'] || {})