In Rails, how to get the selected value from the collection_select helper method?

Let's say I have on my controller:

@parts = Part.all

and on the view form:

<%= f.collection_select(:part_id, @parts, :id, :title, { :prompt => true } %>

It's working for the new/create actions, edit action works too but I would like to know how to get the selected value for doing a find on another model, since it's a nested form.

Or the only way is get it using some javascript?

Doing:

<%= debug f.object.part %>

I get this:

--- !ruby/object:Part
attributes:
id: 1
title: Pearl 02
part_type_id: 36
created_at: 2011-07-28 07:52:09.000000000Z
updated_at: 2011-07-28 08:34:02.000000000Z
set: !!null 
price: 3.53
 changed_attributes: {}
 previously_changed: {}
 attributes_cache: {}
 marked_for_destruction: false
 destroyed: false
 readonly: false
 new_record: false

Which the only attribute I want for now is "price".

Have tried to access the value doing:

<%= f.object.part.price %>

but then it returns an error:

undefined method `price' for nil:NilClass

the trace shows me something related to an helper method I have also.

app/views/items/_item_part_fields.html.erb:7:in `_app_views_items__item_part_fields_html_erb___1683960156823546996_2175548540_683257483947746572'
app/helpers/application_helper.rb:46:in `block in link_to_add_fields'
app/helpers/application_helper.rb:45:in `link_to_add_fields'

the helper method that causes this error:

def link_to_add_fields(name, f, association)
  new_object = f.object.class.reflect_on_association(association).klass.new
  fields = f.fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
    render(association.开发者_JAVA技巧to_s.singularize + "_fields", :f => builder)
end
  link_to_function(name, "add_fields(this, '#{association}', '#{escape_javascript(fields)}')" )
end  

Removing the link to this helper method and calling f.object.part.price works.

What you think?

<% f.object.part_id = XXX %>
<%= f.collection_select(:part_id, @parts, :id, :title, { :prompt => true } %>

Or better to set this stuff in controller (since it is nested form I can't write solution with controller)