How to create association for nested model in a form - ruby-on-rails

Question

In my Ruby on Rails application I want to allow the adding/editing of a nested model which itself has an associated model.
model Survey
string title
has_many questions
model Question
string question
belongs_to category
model Category
string name
For the sake of argument let's assume that the user should always have to enter a new category when entering a question (I couldn't come up with a better example, sigh).
In my model/survey/edit.html.erb I have a working setup for adding questions and saving them. However when I added the Category model to the picture, I now face the problem that when adding a new Question, there is no corresponding Category name-field displayed. I suspect this is because even though I do call Question.new, I do not call question.category.build - and I have no idea where/how to do that.
My edit.html.erb:
<h1>Editing Survey</h1>
<%= render :partial => 'form' %>
My _form.html.erb:
<% form_for(#survey) do |f| %>
<%= f.error_messages %>
<p>
<%= f.label :title %><br />
<%= f.text_field :title %>
</p>
<div id="questions">
<% f.fields_for :questions do |q| %>
<%= render :partial => 'question', :locals => { :pf => q } %>
<% end %>
</div>
<%= add_a_new_child_link("New question", f, :questions) %>
<% end %>
My _question.html.erb:
<div class="question">
<%= pf.label :question %>
<%= pf.text_field :question %>
<% pf.fields_for :category do |c| %>
<p>
<%= c.label :name, "Category:" %>
<%= c.text_field :name %>
</p>
<% end %>
</div>

Answer 1

A quick fix for your situation is to use virtual attributes.
EG, in your Question model:
def category_name=(new_name)
if category then
category.name = new_name
else
category = Category.new(:name => new_name)
end
end
def category_name
return category.name if category
""
end
In your _question, there no need to use nested form. Just add something like:
<%= pf.text_field :category_name %>
I didn't test it, but you probably caught the ideea.