Rails 3 + Ajax: how to access my local form builder instance

Question

I have a form that displays a set of inputs. I also have a button, and when clicked, I make an ajax request which is supposed to replace the existing inputs with a different set

Answer 1

Can you use fields_for in your partial, and pass the @object to it? That way you don't need to pass a form builder?

partial:

<%= fields_for object do |f| %>
  f.text_field :field_name
<% end %>

$('#info').html("<%= escape_javascript(render 'my_second_fields_partial', object: @object) %>

Answer 2

Many thanks to @flyfish, his answer helped me solved how to do ajax with nested attributes. I took @flynfish answer and tweeked it for my situation:

<%= fields_for object do |f| %>
  <%= f.fields nested_object, child_index: Time.now.to_i do |builder| %>
<% end %>

$('#info').html("<%= escape_javascript(render 'my_second_fields_partial', object: @object), nested_object: @nested_object %>

The child_index is important for without it your params hash will build with [0] and will then overlay any other records built with the initial fields_for which starts at zero and increments from there.