title: Rails 3 form_for and fields_for issue
body: <p>I have the following model:</p>
  
  <pre><code>class Employee &lt; ActiveRecord::Base
  has_many :emp_assignments, :dependent =&gt; :destroy, :order =&gt; 'created_at DESC'
  has_many :active_assignments, :class_name =&gt; "EmpAssignment", :conditions=&gt;"end_date is not null or end_date &gt; CURRENT DATE", :order =&gt; 'created_at DESC'
  </code></pre>
  
  <p>Form employees/edit_emp_projects.html.erb</p>
  
  <pre><code>&lt;%= form_for(@employee, :html =&gt; {:class =&gt; "employee_project_form"}) do |f| %&gt;
  
    &lt;%= f.fields_for :active_assignments do |eaf| %&gt;
      &lt;%= render 'emp_assignments/form_emp_assnmnt', :eaf=&gt;eaf %&gt;
    &lt;% end %&gt; 
  
  &lt;% end %&gt;
  </code></pre>
  
  <p>Problem is inside emp_assignments/_form_emp_assnmnt.html.erb:</p>
  
  <pre><code>&lt;%if eaf.object.new_record? %&gt;
  
    &lt;%= render 'emp_assignments/form_segment', :eaf=&gt;eaf %&gt;
  
  &lt;% else %&gt;
  
    &lt;%= eaf.hidden_field :product_id %&gt;
  
  &lt;% end %&gt;
  </code></pre>
  
  <p>Rails complains that eaf.object is nil, failing by saying undefined method new_record? on nil class.</p>
  
  <p>If I change :active_assignments in edit_emp_projects.html.erb to :emp_assignments the problem goes away. What's going wrong when I refer to :active_assignments?</p>