From 04f84470132f5c326d8e9a59b4047ae4076bcc96 Mon Sep 17 00:00:00 2001
From: David Yip <yipdw@northwestern.edu>
Date: Sun, 8 Mar 2009 22:27:57 -0500
Subject: [PATCH] Don't yield a form builder for nil one-to-one associations.

Also added documentation on behavior of one-to-one association builders on
associations that evaluate to nil.
---
 actionpack/lib/action_view/helpers/form_helper.rb |   20 +++++++++++++++++++-
 actionpack/test/template/form_helper_test.rb      |   17 +++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletions(-)

diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index a589bcb..0c54f6c 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -403,6 +403,19 @@ module ActionView
       #     <% end %>
       #   <% end %>
       #
+      # Form builders on models obtained from one-to-one associations only
+      # execute when the underlying model objects actually exist.  In this
+      # example, if +address+ were nil, then the address builder would have
+      # yielded no output.  To ensure that your forms are rendered, you must
+      # make sure that your model objects exist when appropriate:
+      #
+      #   class Person
+      #     def initialize
+      #       @address = Address.new
+      #     end
+      #     ...
+      #   end
+      #
       # ==== One-to-many
       #
       # Consider a Person class which returns an _array_ of Project instances
@@ -1010,7 +1023,12 @@ module ActionView
               fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index}]", child, args, block)
             end.join
           else
-            fields_for_nested_model(name, explicit_object || association, args, block)
+            object = explicit_object || association
+            if object
+              fields_for_nested_model(name, object, args, block)
+            else
+              ""
+            end
           end
         end
 
diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index 654eee4..b39cec1 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -473,6 +473,23 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_nested_fields_for_does_not_yield_builder_on_nil_association
+    @post.author = nil
+
+    form_for('post', @post) do |f|
+      concat f.text_field(:title)
+      f.fields_for :author do |c|
+        concat c.text_field(:name)
+      end
+    end
+
+    expected = '<form action="http://www.example.com" method="post">' +
+               '<input name="post[title]" size="30" type="text" id="post_title" value="Hello World" />' +
+               '</form>'
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_nested_fields_for_with_index_and_parent_fields
     form_for('post', @post, :index => 1) do |c|
       concat c.text_field(:title)
-- 
1.6.1