Add test cases for nil polymorphic associations

lib/active_model/serializer.rb

@@ -131,10 +131,12 @@ def serialize(object, scope, context, options)
 
 
         def serialize_ids(object, scope)
-          if polymorphic?
+          if polymorphic? && object
             { 
               polymorphic_key(object) => object.read_attribute_for_serialization(:id),
             }
+          elsif polymorphic? && !object
+            { }
           elsif object
             { key => object.read_attribute_for_serialization(:id) }
           else

test/serializer_test.rb

@@ -667,6 +667,25 @@ def test_polymorphic_has_one
     }, serializer.as_json)
   end
 
+  def test_polymorphic_has_one_with_nil
+    polymorphic_blog = Class.new do
+      attr_accessor :writer
+    end
+
+    polymorphic_serializer = Class.new(ActiveModel::Serializer) do
+      has_one :writer, :polymorphic => true
+    end
+
+    user = PolymorphicUser.new
+    blog = polymorphic_blog.new
+    blog.writer = nil
+
+    serializer = polymorphic_serializer.new(blog, user)
+
+    assert_equal({
+    }, serializer.as_json)
+  end
+
   def test_polymorphic_has_one_with_ids
     polymorphic_blog = Class.new do
       attr_accessor :writer
@@ -688,6 +707,26 @@ def test_polymorphic_has_one_with_ids
     }, serializer.as_json)
   end
 
+  def test_polymorphic_has_one_id_with_nil
+    polymorphic_blog = Class.new do
+      attr_accessor :writer
+    end
+
+    polymorphic_serializer = Class.new(ActiveModel::Serializer) do
+      embed :ids
+      has_one :writer, :polymorphic => true
+    end
+
+    user = PolymorphicUser.new
+    blog = polymorphic_blog.new
+    blog.writer = nil
+
+    serializer = polymorphic_serializer.new(blog, user)
+
+    assert_equal({
+    }, serializer.as_json)
+  end
+
   def test_root_provided_in_options
     author_serializer = Class.new(ActiveModel::Serializer) do
       attributes :id, :name