add namespace options to form_for

You can provide a namespace for your form to ensure uniqueness of id attributes on form elements.
The namespace attribute will be prefixed with underscore on the generate HTML id

actionpack/lib/action_view/helpers/form_helper.rb

@@ -158,6 +158,9 @@ def convert_to_model(object)
       # * <tt>:url</tt> - The URL the form is submitted to. It takes the same
       #   fields you pass to +url_for+ or +link_to+. In particular you may pass
       #   here a named route directly as well. Defaults to the current action.
+      # * <tt>:namespace</tt> - A namespace for your form to ensure uniqueness of
+      #   id attributes on form elements. The namespace attribute will be prefixed
+      #   with underscore on the generate HTML id.
       # * <tt>:html</tt> - Optional HTML attributes for the form tag.
       #
       # Also note that +form_for+ doesn't create an exclusive scope. It's still
@@ -385,7 +388,7 @@ def apply_form_for_options!(object_or_array, options) #:nodoc:
         action, method = object.respond_to?(:persisted?) && object.persisted? ? [:edit, :put] : [:new, :post]
         options[:html].reverse_merge!(
           :class  => as ? "#{as}_#{action}" : dom_class(object, action),
-          :id     => as ? "#{as}_#{action}" : dom_id(object, action),
+          :id     => as ? "#{as}_#{action}" : [options[:namespace], dom_id(object, action)].compact.join("_").presence,
           :method => method
         )
 
@@ -971,6 +974,7 @@ class InstanceTag
       def initialize(object_name, method_name, template_object, object = nil)
         @object_name, @method_name = object_name.to_s.dup, method_name.to_s.dup
         @template_object = template_object
+
         @object_name.sub!(/\[\]$/,"") || @object_name.sub!(/\[\]\]$/,"]")
         @object = retrieve_object(object)
         @auto_index = retrieve_autoindex(Regexp.last_match.pre_match) if Regexp.last_match
@@ -989,6 +993,7 @@ def to_label_tag(text = nil, options = {}, &block)
 
         add_default_name_and_id_for_value(tag_value, name_and_id)
         options.delete("index")
+        options.delete("namespace")
         options["for"] ||= name_and_id["id"]
 
         if block_given?
@@ -1195,6 +1200,7 @@ def add_default_name_and_id(options)
             options["name"] ||= tag_name + (options['multiple'] ? '[]' : '')
             options["id"] = options.fetch("id"){ tag_id }
           end
+          options["id"] = [options.delete('namespace'), options["id"]].compact.join("_").presence
         end
 
         def tag_name
@@ -1253,7 +1259,7 @@ def initialize(object_name, object, template, options, proc)
         @nested_child_index = {}
         @object_name, @object, @template, @options, @proc = object_name, object, template, options, proc
         @parent_builder = options[:parent_builder]
-        @default_options = @options ? @options.slice(:index) : {}
+        @default_options = @options ? @options.slice(:index, :namespace) : {}
         if @object_name.to_s.match(/\[\]$/)
           if object ||= @template.instance_variable_get("@#{Regexp.last_match.pre_match}") and object.respond_to?(:to_param)
             @auto_index = object.to_param
@@ -1280,6 +1286,7 @@ def fields_for(record_name, record_object = nil, fields_options = {}, &block)
         fields_options, record_object = record_object, nil if record_object.is_a?(Hash) && record_object.extractable_options?
         fields_options[:builder] ||= options[:builder]
         fields_options[:parent_builder] = self
+        fields_options[:namespace] = fields_options[:parent_builder].options[:namespace]
 
         case record_name
         when String, Symbol

actionpack/test/template/form_helper_test.rb

@@ -935,6 +935,82 @@ def test_form_for_with_nil_index_option_override
     assert_dom_equal expected, output_buffer
   end
 
+  def test_form_for_with_namespace
+    form_for(@post, :namespace => 'namespace') do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.check_box(:secret)
+    end
+
+    expected = whole_form('/posts/123', 'namespace_edit_post_123', 'edit_post', 'put') do
+      "<input name='post[title]' size='30' type='text' id='namespace_post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='namespace_post_body' rows='20' cols='40'>Back to the hill and over it again!</textarea>" +
+      "<input name='post[secret]' type='hidden' value='0' />" +
+      "<input name='post[secret]' checked='checked' type='checkbox' id='namespace_post_secret' value='1' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_form_for_with_namespace_with_label
+    form_for(@post, :namespace => 'namespace') do |f|
+      concat f.label(:title)
+      concat f.text_field(:title)
+    end
+
+    expected = whole_form('/posts/123', 'namespace_edit_post_123', 'edit_post', 'put') do
+      "<label for='namespace_post_title'>Title</label>" +
+      "<input name='post[title]' size='30' type='text' id='namespace_post_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
+  def test_two_form_for_with_namespace
+    form_for(@post, :namespace => 'namespace_1') do |f|
+      concat f.label(:title)
+      concat f.text_field(:title)
+    end
+
+    expected_1 = whole_form('/posts/123', 'namespace_1_edit_post_123', 'edit_post', 'put') do
+      "<label for='namespace_1_post_title'>Title</label>" +
+      "<input name='post[title]' size='30' type='text' id='namespace_1_post_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected_1, output_buffer
+
+    form_for(@post, :namespace => 'namespace_2') do |f|
+      concat f.label(:title)
+      concat f.text_field(:title)
+    end
+
+    expected_2 = whole_form('/posts/123', 'namespace_2_edit_post_123', 'edit_post', 'put') do
+      "<label for='namespace_2_post_title'>Title</label>" +
+      "<input name='post[title]' size='30' type='text' id='namespace_2_post_title' value='Hello World' />"
+    end
+
+    assert_dom_equal expected_2, output_buffer
+  end
+
+  def test_fields_for_with_namespace
+    @comment.body = 'Hello World'
+    form_for(@post, :namespace => 'namespace') do |f|
+      concat f.text_field(:title)
+      concat f.text_area(:body)
+      concat f.fields_for(@comment) { |c|
+        concat c.text_field(:body)
+      }
+    end
+
+    expected = whole_form('/posts/123', 'namespace_edit_post_123', 'edit_post', 'put') do
+      "<input name='post[title]' size='30' type='text' id='namespace_post_title' value='Hello World' />" +
+      "<textarea name='post[body]' id='namespace_post_body' rows='20' cols='40'>Back to the hill and over it again!</textarea>" +
+      "<input name='post[comment][body]' size='30' type='text' id='namespace_post_comment_body' value='Hello World' />"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_submit_with_object_as_new_record_and_locale_strings
     old_locale, I18n.locale = I18n.locale, :submit