How to get correct index and name with fields_for parents[], parent do |f| and f.fields_for :children, child

Hi,    I'm editing multiple instances of a parent model in an index view in one form, as in Railscasts #198. Each parent has_many :children and accepts_nested_attributes_for :children, as in Railscasts #196 and #197

<%= form_tag %>   <% for parent in @parents %>     <%= fields_for "parents", parent do |f|       <%= f.text_field :job %>       <%= f.fields_for :children do |cf| %>          <% cf.text_field :chore %>       <% end %>     <% end %>   <% end %> <% end %>

Given parent.id==1 f.text_field :job correctly generates

<input id="parents_1_job" type="text" value="coding" size="30" name="parents[1][job]">

But cf.text_field :chore generates ids and names that don't have the parent index.

id="parents_children_attributes_0_chore" name="parents[children_attributes][0][chore]"

I've stared at the tail end of http://guides.rubyonrails.org/form_helpers.html and tried lots of combos but haven't found the magic syntax. Do I need to subclass FormBuilder and teach it what to do somehow, or is there a way to tell fields_for what it needs?

Thanks