nil_class when using partial - ruby-on-rails

Question

I have the following code:
new.html.erb
<% form_for #car,:html => {:name => 'car_form'} do |f| %>
<% time_created = Time.new %>
<%= f.hidden_field :CAR_DATE_CREATED, :value => time_created %>
<%= render :partial => "car_partial", :locals => {:object => #car } %>
<%= f.submit 'Create' %>
<% end %>
_car_partial.html.erb
<% fields_for #car do |f| %>
Origin: <%= f.select(:ORIGIN_TYPE,
[['Origin X', 'X'],
['Origin Y', 'Y'],
['Other origin', 'Other']
],{:prompt => "Please select"}
) %>
<%= observe_field("car_ORIGIN_TYPE", :frequency => 2,
:url => { :controller => 'car',
:action => :display_other_origin },
:with => "'id='+value") %>
<span id="otherOrigin" > </span>
<% end %>
controller code
def display_other_origin
origin = params[:id]
end
display_other_origin.rjs
if params[:id] == "Other"
page['otherOrigin'].replace_html :partial => 'car/other_origin_type', :locals => {:car => #car }
else
page.replace_html :otherOrigin, ''
end
_other_origin_type.html.erb
<% fields_for #car do |f| %>
If other, please state: <%= f.text_field :ORIGIN_TYPE, :size => 20%>
<% end %>
If user selects 'Other origin', the partial is displayed correctly but when i do <view source>,
Origin: <select id="car_ORIGIN_TYPE" name="car[ORIGIN_TYPE]">
<option value="X">Origin X</option>
<option value="Y">Origin Y</option>x
<option value="Other">Other origin</option>
</select>
<script type="text/javascript">
//<![CDATA[
new Form.Element.Observer('car_ORIGIN_TYPE', 2, function(element, value) {new Ajax.Request('/cars/display_other_origin', {asynchronous:true, evalScripts:true, parameters:'id='+value + '&authenticity_token=' + encodeURIComponent('+MtiIQj/KfX2huaetD1E7W7f2pu/HC2d31BhCPjcmlQ=')})})
//]]>
</script>
<span id="otherOrigin">
If other, please state: <input id="nil_class_ORIGIN_TYPE" name="nil_class[ORIGIN_TYPE]" size="20" type="text">
</span>
nil_class is displayed instead of 'car'.
Any help???
Thanks

Answer 1

Your controller action display_other_origin is not setting the #car variable - so it is nil when used in the display_other_origin.rjs file.
Basically, in your observe_field method you need to also set a parameter for the car id. For example:
<%= observe_field("car_ORIGIN_TYPE", :frequency => 2,
:url => { :controller => 'car',
:action => :display_other_origin },
:with => "'id='+value+'&car_id='#{#car.id}") %>
Then you can do #car = Car.find(param[:car_id]) in display_other_origin
...although I should note that convention states that params[:id] - when passed to the CarsController - should be the car id, and your origin should be params[:origin]