Howto : work with index in Rails 3's fields_for

index-with-fields_for.html.erb

<%= form_for parent do |parent_form_builder| %>
  <%= parent_form_builder.text_field :name %>
  <% parent.children.each_with_index do |child, index| %>
    <% parent_form_builder.fields_for :children, child do |child_form_builder| %> 
      <%= child_form_builder.select :age, (0..99).to_a %>
      <%# generates "parent[:children_attributes][index][age]" as name for the input %>
    <% end %>
  <% end %>
  <%= f.submit %>
<% end %>

Wanting to put all the response text fields into a tab system (from foundation). What is the best way to do this?

= item_step_fields.fields_for :responses do |response_fields, i|

div.show.button
fieldset.response.translatable

ul-tabs-content
  li-simple#{i}Tab
    = response_fields.label :response_text, "Response Text"
    = response_fields.text_area :response_text, rows: "5", class: "wysiwyg", style: 'width:100%'
    = render partial: 'readable_text/form_fields', locals: { fields: response_fields, style: 'width:100%' }
    = response_fields.text_field :sort_order, class: "input-text small", placeholder: "Sort Order"
  div
    = render partial: 'images/form_fields', locals: { fields: response_fields }
  div.correctness
    = response_fields.label :is_correct do
      = response_fields.check_box :is_correct, class: "is_correct"
      = "Correct"
    div.rationale
      = response_fields.label :rationale
      = response_fields.text_area :rationale, rows: "5", style: 'width:100%'

The main idea is to iterate over each child object

= question.responses.each_with_index do |response, i|

then, inside this loop, to build each mini-form for these children, giving the child you want to deal with to the builder

   = question_form_builder.fields_for :responses, response do |response_form_builder|
     = response_form_builder.check_box :is_correct

Hope it helps, but I'm not so familiar with haml nor foundation so implementation may depends regarding what you want to do :) I can help with questions, but it's hard for me to deal with this kind of rough stuff blindly.