diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index a4f5116a5d..32630e5ded 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Add `index` method to FormBuilder class. *Jorge Bejar*
+
 *   Remove the leading \n added by textarea on assert_select. *Santiago Pastorino*
 
 *   Changed default value for `config.action_view.embed_authenticity_token_in_remote_forms`
diff --git a/actionpack/lib/action_view/helpers/form_helper.rb b/actionpack/lib/action_view/helpers/form_helper.rb
index 6219a7a924..3f5829d86a 100644
--- a/actionpack/lib/action_view/helpers/form_helper.rb
+++ b/actionpack/lib/action_view/helpers/form_helper.rb
@@ -673,6 +673,19 @@ module ActionView
       #     <% end %>
       #     ...
       #   <% end %>
+      #
+      # When a collection is used you might want to know the index of each
+      # object into the array. For this purpose, the <tt>index</tt> method
+      # is available in the FormBuilder object.
+      #
+      #   <%= form_for @person do |person_form| %>
+      #     ...
+      #     <%= person_form.fields_for :projects do |project_fields| %>
+      #       Project #<%= project_fields.index %>
+      #       ...
+      #     <% end %>
+      #     ...
+      #   <% end %>
       def fields_for(record_name, record_object = nil, options = {}, &block)
         builder = instantiate_builder(record_name, record_object, options)
         output = capture(builder, &block)
@@ -1038,7 +1051,7 @@ module ActionView
 
       attr_accessor :object_name, :object, :options
 
-      attr_reader :multipart, :parent_builder
+      attr_reader :multipart, :parent_builder, :index
       alias :multipart? :multipart
 
       def multipart=(multipart)
@@ -1076,6 +1089,7 @@ module ActionView
           end
         end
         @multipart = nil
+        @index = options[:index] || options[:child_index]
       end
 
       (field_helpers - [:label, :check_box, :radio_button, :fields_for, :hidden_field, :file_field]).each do |selector|
@@ -1107,12 +1121,14 @@ module ActionView
         end
 
         index = if options.has_key?(:index)
-          "[#{options[:index]}]"
+          options[:index]
         elsif defined?(@auto_index)
           self.object_name = @object_name.to_s.sub(/\[\]$/,"")
-          "[#{@auto_index}]"
+          @auto_index
         end
-        record_name = "#{object_name}#{index}[#{record_name}]"
+
+        record_name = index ? "#{object_name}[#{index}][#{record_name}]" : "#{object_name}[#{record_name}]"
+        fields_options[:child_index] = index
 
         @template.fields_for(record_name, record_object, fields_options, &block)
       end
@@ -1250,7 +1266,8 @@ module ActionView
             explicit_child_index = options[:child_index]
             output = ActiveSupport::SafeBuffer.new
             association.each do |child|
-              output << fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index(name)}]", child, options, block)
+              options[:child_index] = nested_child_index(name) unless explicit_child_index
+              output << fields_for_nested_model("#{name}[#{options[:child_index]}]", child, options, block)
             end
             output
           elsif association
diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index f13296a175..3fa3898ec8 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -1835,6 +1835,56 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_nested_fields_for_index_method_with_existing_records_on_a_nested_attributes_collection_association
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_for(@post) do |f|
+      expected = 0
+      @post.comments.each do |comment|
+        f.fields_for(:comments, comment) { |cf|
+          assert_equal cf.index, expected
+          expected += 1
+        }
+      end
+    end
+  end
+
+  def test_nested_fields_for_index_method_with_existing_and_new_records_on_a_nested_attributes_collection_association
+    @post.comments = [Comment.new(321), Comment.new]
+
+    form_for(@post) do |f|
+      expected = 0
+      @post.comments.each do |comment|
+        f.fields_for(:comments, comment) { |cf|
+          assert_equal cf.index, expected
+          expected += 1
+        }
+      end
+    end
+  end
+
+  def test_nested_fields_for_index_method_with_existing_records_on_a_supplied_nested_attributes_collection
+    @post.comments = Array.new(2) { |id| Comment.new(id + 1) }
+
+    form_for(@post) do |f|
+      expected = 0
+      f.fields_for(:comments, @post.comments) { |cf|
+        assert_equal cf.index, expected
+        expected += 1
+      }
+    end
+  end
+
+  def test_nested_fields_for_index_method_with_child_index_option_override_on_a_nested_attributes_collection_association
+    @post.comments = []
+
+    form_for(@post) do |f|
+      f.fields_for(:comments, Comment.new(321), :child_index => 'abc') { |cf|
+        assert_equal cf.index, 'abc'
+      }
+    end
+  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)]