do what the superclass does in the case that objects do not match

activerecord/lib/active_record/core.rb

@@ -312,6 +312,8 @@ module ActiveRecord
     def <=>(other_object)
       if other_object.is_a?(self.class)
         self.to_key <=> other_object.to_key
+      else
+        super
       end
     end
 

activerecord/test/cases/base_test.rb

@@ -579,6 +579,14 @@ class BasicsTest < ActiveRecord::TestCase
     assert_nil topic <=> category
   end
 
+  def test_comparison_with_different_objects_in_array
+    topic = Topic.create
+    category = Category.create(:name => "comparison")
+    assert_raises(ArgumentError) do
+      [1, topic].sort
+    end
+  end
+
   def test_readonly_attributes
     assert_equal Set.new([ 'title' , 'comments_count' ]), ReadonlyTitlePost.readonly_attributes