Dont try to load the record from the db if preloading didn't find any…

…thing

activerecord/lib/active_record/association_preload.rb

@@ -157,6 +157,11 @@ def set_association_single_records(id_to_record_map, reflection_name, associated
             association_proxy.__send__(:set_inverse_instance, associated_record, mapped_record)
           end
         end
+
+        id_to_record_map.each do |id, records|
+          next if seen_keys.include?(id.to_s)
+          records.each {|record| record.send("set_#{reflection_name}_target", nil) }            
+        end
       end
 
       # Given a collection of ActiveRecord objects, constructs a Hash which maps

activerecord/test/cases/associations/eager_test.rb

@@ -830,5 +830,11 @@ def test_include_has_one_using_primary_key
       assert_equal expected, firm.account_using_primary_key
     end
   end
-
+
+  def test_preloading_empty_polymorphic_parent
+    t = Tagging.create!(:taggable_type => 'Post', :taggable_id => Post.maximum(:id) + 1, :tag => tags(:general))
+
+    assert_queries(2) { @tagging = Tagging.preload(:taggable).find(t.id) }
+    assert_no_queries { assert ! @tagging.taggable }
+  end
 end