fields_for has_many not showing as array on form

Question

Typically when I use a fields_for on a simple has_many association, the input names on the form look something like foo[bars_attributes][0][name], but in my case, it's not showing up as an array on the form - instead I get something like foo[bars][name], which will blow up when I try to submit because it's trying to assign to a name property of an array - and I can't figure out what's different from other cases. Here's the code, which has obviously been simplified and renamed for this venue.

My model:

  class Foo
    has_many :bars
  end

The controller:

  class FooController < ApplicationController
    def new
      @foo = Foo.new
      @foo.bars.build
      @foo
    end
  end

The view:

<div>
  <%= form_for @foo do |f| %>
    <%= f.fields_for :bars do |bar_fields| %>
      <%= bar_fields.text_field :name %>
    <% end %>
    <div class="single_column">
      <%= f.submit "Submit" %>
    </div>
  <% end %>
</div>

The resulting markup:

<input name="foo[bars][name]" id="foo_bars_name" />

Answer 1

Perhaps Foo could merit with having accepts_nested_attributes_for :bars in it?