How to get index in partial with fields_for on nested records [duplicate]

Question

Given:

<%= f.fields_for :socials do |builder| %>
  <%= render :partial => "social_fields", :locals => { :f => builder} %>
<% end %>

how can I get a counter/index inside the _social_fields.html.erb counter (in order to to something on the last item)? social_fields_counter seems to only be generated if you explicitly pass :collection to render, but these are nested objects with accepts_nested_attributes_for :socials in the parent.

Answer 1

Inside _social_fields.html.erb:

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