diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index 6d6d623..e1f8b4a 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -881,8 +881,9 @@ module ActionView
 
         def add_default_name_and_id(options)
           if options.has_key?("index")
-            options["name"] ||= tag_name_with_index(options["index"])
-            options["id"]   ||= tag_id_with_index(options["index"])
+            index = (options["index"].is_a?(Symbol) && @object.respond_to?(options["index"])) ? @object.method(options["index"]).call : options["index"]
+            options["name"] ||= tag_name_with_index(index)
+            options["id"]   ||= tag_id_with_index(index)
             options.delete("index")
           elsif defined?(@auto_index)
             options["name"] ||= tag_name_with_index(@auto_index)
@@ -953,7 +954,7 @@ module ActionView
 
       def fields_for(record_or_name_or_array, *args, &block)
         if options.has_key?(:index)
-          index = "[#{options[:index]}]"
+          index = (options[:index].is_a?(Symbol) && @object.respond_to?(options[:index])) ? "[#{@object.method(options[:index]).call}]" : "[#{options[:index]}]"
         elsif defined?(@auto_index)
           self.object_name = @object_name.to_s.sub(/\[\]$/,"")
           index = "[#{@auto_index}]"
@@ -1029,6 +1030,7 @@ module ActionView
             explicit_child_index = args.last[:child_index] if args.last.is_a?(Hash)
 
             children.map do |child|
+              explicit_child_index = child.method(explicit_child_index).call if explicit_child_index.is_a?(Symbol) && child.respond_to?(explicit_child_index)
               fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index(name)}]", child, args, block)
             end.join
           else
diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index 515f73c..29bda5f 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -787,6 +787,23 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_nested_fields_for_with_child_method_index_option_override_on_a_nested_attributes_collection_association
+    @post.comments = []
+
+    form_for(:post, @post) do |f|
+      f.fields_for(:comments, Comment.new(321), :child_index => :id) do |cf|
+        concat cf.text_field(:name)
+      end
+    end
+
+    expected = '<form action="http://www.example.com" method="post">' +
+               '<input id="post_comments_attributes_321_id" name="post[comments_attributes][321][id]" type="hidden" value="321" />' +
+               '<input id="post_comments_attributes_321_name" name="post[comments_attributes][321][name]" size="30" type="text" value="comment #321" />' +
+               '</form>'
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_nested_fields_uses_unique_indices_for_different_collection_associations
     @post.comments = [Comment.new(321)]
     @post.tags = [Tag.new(123), Tag.new(456)]
@@ -896,6 +913,22 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_fields_for_with_method_index_option_override
+    fields_for("post[]", @post, :index => :id) do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+    
+    expected =
+      "<input name='post[123][title]' size='30' type='text' id='post_123_title' value='Hello World' />" +
+      "<textarea name='post[123][body]' id='post_123_body' rows='20' cols='40'>Back to the hill and over it again!</textarea>" +
+      "<input name='post[123][secret]' type='hidden' value='0' />" +
+      "<input name='post[123][secret]' checked='checked' type='checkbox' id='post_123_secret' value='1' />"
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_fields_for_without_object
     fields_for(:post) do |f|
       concat f.text_field(:title)