Rails: fields_for with index?

The answer is quite simple as the solution is provided within Rails. You can use f.options params. So, inside your rendered _some_form.html.erb,

Index can be accessed by:

<%= f.options[:child_index] %>

You don't need to do anything else.

---

Update: It seems that my answer wasn't clear enough...

Original HTML File:

<!-- Main ERB File -->
<% f.fields_for :questions do |builder| %>  
  <%= render 'some_form', :f => builder %>
<% end %>

Rendered Sub-Form:

<!-- _some_form.html.erb -->
<%= f.options[:child_index] %>

The answer below was posted many years ago, for a modern approach see: https://stackoverflow.com/a/22640703/105403

---

This would actually be a better approach, following Rails documentation more closely:

<% @questions.each.with_index do |question,index| %>
    <% f.fields_for :questions, question do |fq| %>  
        # here you have both the 'question' object and the current 'index'
    <% end %>
<% end %>

From: http://railsapi.com/doc/rails-v3.0.4/classes/ActionView/Helpers/FormHelper.html#M006456

It’s also possible to specify the instance to be used:

  <%= form_for @person do |person_form| %>
    ...
    <% @person.projects.each do |project| %>
      <% if project.active? %>
        <%= person_form.fields_for :projects, project do |project_fields| %>
          Name: <%= project_fields.text_field :name %>
        <% end %>
      <% end %>
    <% end %>
  <% end %>

As of Rails 4.0.2, an index is now included in the FormBuilder object:

https://api.rubyonrails.org/classes/ActionView/Helpers/FormBuilder.html#method-i-fields_for

For example:

<%= form_for @person do |person_form| %>
  ...
  <%= person_form.fields_for :projects do |project_fields| %>
    Project #<%= project_fields.index %>
  ...
  <% end %>
  ...
<% end %>