FormBuilder in rjs

I have a page, which renders a partial that contains a 'State' drop down list. Upon change, it re-renders the 'City' drop down, re- populating it based on selected state_id:

Page: /customer/new.rhtml

<% form_for :customer, :url => customers_path do |f| %>

State: <%= f.collection_select :state_id, State.find(:all), 'id', 'description' %> <%= observe_field :customer_state_id, :url => search_cities_url, :with => 'search[state_id]' %>

<.div id='state_div'><%= render :partial => 'city/list', :object => , :locals => { :f => f } %>

<% end %>

class CustomerController...   def create     @customer = Customer.create params[:customer] # => { :state_id => X, :city_id => Y }     ...   end end

Partial: /city/_list.rhtml

City: <%= f.collection_select :city_id, list, 'id', 'name' %>

class CityController...   def search     @cities = City.find :all, :conditions => params[:search] # => { :state_id => X }   end

RJS: /city/search.rjs

page.replace_html 'state_div', :partial => 'list', :object => @cities, :locals => { :f => ??? }

My problem is that when rendering my partial 'city/list' from new.rhtml, I have access to my FormBuilder f and therefore can DRY up my params hash key 'customer' for all form input elements I use in my partial, which is great. However, when I render my partial from an rjs, I no longer have access to the FormBuilder f. I don't want to hard-code the controller#params key 'search' inside my partial either because I can have other rhtml pages that uses the same 'city/list' partial but want to render it inside a different form_for key than :customer.

So here is my question: for my partial 'city/_list.rhtml', how can I customize its controller#params POST key (eg. customer[:city_id]) such that I can have multiple render entry points into the partial but not all has the good-ole FormBuilder?