Fix order method to accept relation attributes.

Signed-off-by: José Valim <jose.valim@gmail.com>

activerecord/lib/active_record/relation/query_methods.rb

@@ -137,7 +137,7 @@ module ActiveRecord
 
       arel = arel.group(*@group_values.uniq.select{|g| g.present?}) if @group_values.present?
 
-      arel = arel.order(*@order_values.uniq.select{|o| o.present?}.map(&:to_s)) if @order_values.present?
+      arel = arel.order(*@order_values.uniq.select{|o| o.present?}) if @order_values.present?
 
       selects = @select_values.uniq
 

activerecord/test/cases/relations_test.rb

@@ -503,13 +503,13 @@ class RelationTest < ActiveRecord::TestCase
 
   def test_many
     posts = Post.scoped
-  
+
     assert_queries(2) do
       assert posts.many? # Uses COUNT()
       assert posts.many? {|p| p.id > 0 }
       assert ! posts.many? {|p| p.id < 2 }
     end
-  
+
     assert posts.loaded?
   end
 
@@ -593,4 +593,8 @@ class RelationTest < ActiveRecord::TestCase
     assert_equal "lifo", relation.author
     assert_equal "lifo", relation.limit(1).author
   end
+
+  def test_order_by_relation_attribute
+    assert_equal Post.order(Post.arel_table[:title]).all, Post.order("title").all
+  end
 end