Just a quick post so show you how fields_for form helper can cause invalid HTML when used in combination with accepts_nested_attributes_for in your views.

If you are for any reason editing/creating your child objects in a tabular form, your view might look something like this:

<% form_for(@todo_list) do |f| %>
  <%= f.label      :name %>
  <%= f.text_field :name %>

  <h3>To Do Items:</h3>
  <table>
    <thead>
      <tr>
        <th>To Do Item</th>
        <th>Due Date</th>
        <th>Done?</th>
      </tr>
    </thead>
    <tbody>
      <% f.fields_for :items do |item| %>
        <tr>
          <td><%= item.text_field :content %></td>
          <td><%= item.text_field :due_date %></td>
          <td><%= item.check_box  :done %></td>
        </tr>
      <% end %>
    </tbody>
  </table>
  
  <%= f.submit 'Save' %>
<% end %>

At first glance this looks fine. However, when the ToDoList model has accepts_nested_attributes_for :items, the fields_for helper also outputs a hidden field for each existing Item instance with it’s ID.

This means that we get the following HTML:

<form action="/to_do_lists/1" class="edit_to_do_list" id="edit_to_do_list_1" method="post">
  <div style="margin:0;padding:0;display:inline">
    <input name="_method" type="hidden" value="put" />
    <input name="authenticity_token" type="hidden" value="CE2jyRoewbwsqu4eX8AFWFqsgrMfCN35Jzy6b43MhsA=" />
  </div>
  <label for="to_do_list_name">Name</label>
  <input id="to_do_list_name" name="to_do_list[name]" size="30" type="text" value="Sup" />

  <h3>To Do Items:</h3>
  <table>
    <thead>
      <tr>
        <th>To Do Item</th>
        <th>Due Date</th>
        <th>Done?</th>
      </tr>
    </thead>
    <tbody>
      <input id="to_do_list_items_attributes_0_id" name="to_do_list[items_attributes][0][id]" type="hidden" value="1" />
        <tr>
          <td><input id="to_do_list_items_attributes_0_content" name="to_do_list[items_attributes][0][content]" size="30" type="text" value="arst" /></td>
          <td><input id="to_do_list_items_attributes_0_due_date" name="to_do_list[items_attributes][0][due_date]" size="30" type="text" value="2009-07-30" /></td>
          <td><input name="to_do_list[items_attributes][0][done]" type="hidden" value="0" /><input id="to_do_list_items_attributes_0_done" name="to_do_list[items_attributes][0][done]" type="checkbox" value="1" /></td>
        </tr>
      <input id="to_do_list_items_attributes_1_id" name="to_do_list[items_attributes][1][id]" type="hidden" value="2" />
        <tr>
          <td><input id="to_do_list_items_attributes_1_content" name="to_do_list[items_attributes][1][content]" size="30" type="text" value="arst" /></td>
          <td><input id="to_do_list_items_attributes_1_due_date" name="to_do_list[items_attributes][1][due_date]" size="30" type="text" value="2009-07-30" /></td>
          <td><input name="to_do_list[items_attributes][1][done]" type="hidden" value="0" /><input id="to_do_list_items_attributes_1_done" name="to_do_list[items_attributes][1][done]" type="checkbox" value="1" /></td>
        </tr>
    </tbody>
  </table>
  
  <input id="to_do_list_submit" name="commit" type="submit" value="Save" />
</form>

The keen eye will notice the hidden <input> tags that are direct children of the tbody:

    <tbody>
      <input id="to_do_list_items_attributes_0_id" name="to_do_list[items_attributes][0][id]" type="hidden" value="1" />
        <tr><!-- clip! --></tr>
      <input id="to_do_list_items_attributes_1_id" name="to_do_list[items_attributes][1][id]" type="hidden" value="2" />
        <tr><!-- clip! --></tr>
    </tbody>

This is not a valid place for an <input> tag.

I’ve put an example project demonstrating this on my GitHub

There has been quite an annoying problem that has been bugging @chendo and I today.

In short, fields_for when used with accepts_nested_attributes_for does not reset it’s index when it’s called multiple times for the same attribute.

Our scenario was the following:

We had a 3 levels of hierarchical data that we had to display in a <table>. Because of the nature of the data we were displaying one level as columns and the other level as rows. Unfortunately, HTML mandates that we group everything by rows—each column is really just a single cell. This means that we have to iterate over the “column” values not just once, but once for every row.

Now, fields_for takes care of naming your <input> tags magically with the correct indexes so that your models can accept the params directly and magic happens. However, evidently calling fields_for multiple times was not part of the original intention, and it seems resetting the index after each call was neglected.

@chendo couldn’t see a cleaner way of doing this so we added our own option to reset the index. We didn’t just override the behaviour in case there is a good reason to leave it, but here is our code:

class ActionView::Helpers::FormBuilder
  def fields_for_with_nested_attributes_with_index_reset(association_name, args, block)
    if args.last.is_a?(Hash) && args.last[:reset_index]
      @nested_child_index = nil
    end

    fields_for_without_nested_attributes_without_index_reset(association_name, args, block)
  end

  alias_method_change :fields_for_with_nested_attributes, :index_reset
end