undefined variable 'f' when rendering portions of a form through ajax - ruby-on-rails

Question

I have a situation where a Contact has many Leads and a Lead belongs to Profile polymorphically. I successfully rendered the form as such:
# _form.html.erb
<%= form_for #lead do |f| %>
<%= f.fields_for :contact do |builder| %>
<%= render "contact_fields", :f => builder %>
<%= render "leads_field", f: f %>
<% end %>
<% end %>
# lead_field.html.erb
<%= f.select :practice_type, PRACTICE_TYPES.collect {|type| [ type, type ] }
<%= f.fields_for :practice do |builder| %>
<%= render "#{#practice.class.name.underscore}_field", :f => builder %>
<% end %>
The above works fine with page load. However, a user can select a practice from dropdown menu and then I send ajax request to repopulate the form building an association with that practice:
$('#lead_practice_type').change(function() {
$.ajax({
url: "/leads/new",
data: {
profile_type : $(this).val()
},
dataType: "script"
});
});
The error occurs in new.js.erb:
$("#form_holder").html("<%= escape_javascript(render(:partial => "leads_field"))%>");
I want to only render that partial (if I render whole form, then all their contact information would be erased.). But because that partial contains a local f variable, it blows up:
NameError - undefined local variable or method `f'
I want to maintain the relations between the objects but also update just that one partial. But it appears I cannot do it through the new.js.erb script because when it renders the partial, there is no local variable 'f' passed.
Is there another alternative to achieve what I want?

Answer 1

I found this problem difficult to solve on two fronts. First, There is no way you can identify the form builder from the controller action and javascript. Second, although I am quite familiar with the railscasts episode http://railscasts.com/episodes/196-nested-model-form-revised?view=comments, in that episode he knew the fields_for to generate at compile time, and therefore he hid the markup in the data field of a link. Unfortunately, in my case I would not know the fields_for the user wants until runtime (when the browser loads and user selects an option).
I also realized that fields_for is important to maintain nice associations for form submission so that the create action would be simply: save the parent resource and it saves its associations.
The best solution I could come up with is as follows.
1) Create ALL the polymorphic associations with fields_for and hide the ones the user doesn't want via a select form field:
# template
<div id="runtime-profile">
<% Lead::TYPES.each_with_index do |association, index| %>
<div id="profile_<%= association %>">
<fieldset class="field-border">
<legend class="field-border"><%= association.underscore %></legend>
<%= f.fields_for :practice, build_builder_association(f, association) do |builder| %>
<%= render "#{builder.object.class.name.underscore}_field", :f => builder %>
<% end %>
</div>
<% end %>
</div>
# helper
def build_builder_association(f, association)
f.object.practice_type = association
f.object.build_practice({})
end
Use Javascript to hide and show the templates based on an option selected from dropdown, and then when form is submitted, remove the hidden elements from the DOM:
$("#new_lead").submit(function(){
$("[id^='profile_']:hidden").remove();
})
And that creates the original desired behavior I wanted. However, the solution is far from clean and any better suggestions are welcome.