nested params not being submitted with form - ruby-on-rails

Question

I have a nested form that is based off of the example in this rails cast:
http://railscasts.com/episodes/196-nested-model-form-revised
For my application, I'm trying to let people upload multiple files in a nested form. Each step can have many design files, and steps accept nested attributes for design files:
Step.rb:
class Step < ActiveRecord::Base
attr_accessible :design_files_attributes
has_many :design_files
accepts_nested_attributes_for :design_files, :allow_destroy => :true
...
end
DesignFile.rb:
class DesignFile < ActiveRecord::Base
belongs_to :step
...
end
I'm running into issues when I try to add an additional file after another file has already been added. This is what the form looks like in html:
<div class="upload_new_files">
<fieldset class="design_file_upload_fieldset">
<label for="step_design_files_attributes_0_design_file_path">test.pdf</label>
<input id="step_design_files_attributes_0__destroy" name="step[design_files_attributes][0][_destroy]" type="hidden" value="false">
<i class="icon-trash"></i> Remove File
<input id="step_design_files_attributes_0_step_id" name="step[design_files_attributes][0][step_id]" type="hidden" value="451">
<input id="step_design_files_attributes_0_project_id" name="step[design_files_attributes][0][project_id]" type="hidden" value="120">
<input id="step_design_files_attributes_0_user_id" name="step[design_files_attributes][0][user_id]" type="hidden" value="15">
<hr>
</fieldset>
<fieldset class="design_file_upload_fieldset">
<input id="step_design_files_attributes_70265135732560_design_file_path" name="step[design_files_attributes][70265135732560][design_file_path]" onchange="javascript: addField(this);" type="file" style="display: none;">
<input id="step_design_files_attributes_70265135732560__destroy" name="step[design_files_attributes][70265135732560][_destroy]" type="hidden" value="false">
<i class="icon-trash"></i> Remove File
<input id="step_design_files_attributes_70265135732560_step_id" name="step[design_files_attributes][70265135732560][step_id]" type="hidden" value="446">
<input id="step_design_files_attributes_70265135732560_project_id" name="step[design_files_attributes][70265135732560][project_id]" type="hidden" value="120">
<input id="step_design_files_attributes_70265135732560_user_id" name="step[design_files_attributes][70265135732560][user_id]" type="hidden" value="15">
<hr>
</fieldset>
</div>
But when I submit the form, the design_files_attributes aren't in the parameters. I'm expecting something like this:
Parameters: {"step"=>{ "design_files_attributes"=>{"0"=>{"id"=>"17" ...}}, "70265135732560"=>{...}...}
But, instead, I'm getting this:
Parameters: {"step"=> {"design_files_attributes"=>{"0"=>{"id"=>"17" ...}}
What isn't my new entry being submitted in the form parameters?
Edit
Here is the full params that are being submitted to the controller; I didn't include it above because it includes a lot of other things, but I have it here in case it's useful:
Parameters: {"utf8"=>"✓", "authenticity_token"=>"...=", "step"=>{"name"=>"New Design File", "description"=>"", "question_attributes"=>{"description"=>"", "decision_attributes"=>{"description"=>"", "question_id"=>""}}, "design_files_attributes"=>{"0"=>{"_destroy"=>"false", "step_id"=>"451", "project_id"=>"120", "user_id"=>"15", "id"=>"17"}}, "last"=>"0", "published_on_date"=>"02/04/2014", "published_on_time"=>"05:02 PM", "timeZone"=>"EST"}, "commit"=>"Update Step", "project_id"=>"120", "id"=>"3"}
_step_form.html.erb
<%= semantic_form_for [#project, #step], :html => {:multipart => true} do |f| %>
<%= f.fields_for :design_files, multipart: true do |design_file_form| %>
<%= render 'design_file_fields', f: design_file_form %>
<% end %>
<p class="add_design_file" style="display:none;"><%= link_to_add_fields raw("<icon class='icon-plus-sign' style='color:green;'></icon> Add Another Design File"), f, :design_files %></p>
<% end %>
_design_file_fields.html.erb
<fieldset class="design_file_upload_fieldset">
<% if f.object.new_record? %>
<%= f.file_field :design_file_path, :onChange=>"javascript: addField(this);" %>
<% else %>
<%= f.label :design_file_path, f.object.design_file_path.to_s.split('/').pop() %>
<% end %>
<%= f.hidden_field :_destroy %>
<%= link_to raw('<i class="icon-trash"></i> Remove File'), "#", class: "remove_fields" %>
<%= f.hidden_field :step_id, :value => #step.id %>
<%= f.hidden_field :project_id, :value => #project.id %>
<%= f.hidden_field :user_id, :value => current_user.id %>
<hr>
</fieldset>
steps.js
// add a new design file field to the form
function addField(input) {
console.log('add design field input');
// remove the filefield of recently uploaded file and replace with existing file styling
var filename = $(input).val().split('\\').pop();
$(input).parent('fieldset').prepend('<label>'+filename+'</label>');
$(input).hide();
$('.add_fields').click();
}
application_helper.rb
def link_to_add_fields(name, f, association)
new_object = f.object.send(association).klass.new
id = new_object.object_id
fields = f.fields_for(association, new_object, child_index: id) do |builder|
render(association.to_s.singularize + "_fields", f: builder)
end
link_to(name, '#', class: "add_fields", data: {id: id, fields: fields.gsub("\n", "")})
end

Answer 1

Wow, the answer ended up being totally unrelated to what I had posted! The issue was with the form itself, which had an extra <% end %> element, causing the design files fields_for not being incorporated into the form. I'll leave this response in case it's useful for anyone else.