Allow filters to sort & select based on subvalues (#5622)

Merge pull request 5622

lib/jekyll/filters.rb

@@ -361,7 +361,9 @@ module Jekyll
     private
     def item_property(item, property)
       if item.respond_to?(:to_liquid)
-        item.to_liquid[property.to_s]
+        property.to_s.split(".").reduce(item.to_liquid) do |subvalue, attribute|
+          subvalue[attribute]
+        end
       elsif item.respond_to?(:data)
         item.data[property.to_s]
       else

test/test_filters.rb

@@ -949,6 +949,12 @@ class TestFilters < JekyllUnitTest
         assert_equal [{ "a" => 1 }, { "a" => 2 }, { "b" => 1 }],
           @filter.sort([{ "a" => 2 }, { "b" => 1 }, { "a" => 1 }], "a", "last")
       end
+      should "return sorted by subproperty array" do
+        assert_equal [{ "a" => { "b" => 1 } }, { "a" => { "b" => 2 } },
+                      { "a" => { "b" => 3 } }, ],
+          @filter.sort([{ "a" => { "b" => 2 } }, { "a" => { "b" => 1 } },
+                        { "a" => { "b" => 3 } }, ], "a.b")
+      end
     end
 
     context "to_integer filter" do