class_registrations_controller.rb

class ClassRegistrationsController < InheritedResources::Base
	before_filter :authenticate_user!

	def permitted_params
		params.permit(:class_registration => [
						:person_id, :classdate_id,
						:document_assignments_attributes => [
							:uploaded_document_id,
							:uploaded_document => [
								:document, :owner_type, :owner_id
							]
						]
					])
	end

end

document_assignment.rb

class DocumentAssignment < ActiveRecord::Base
	belongs_to :uploaded_document
	accepts_nested_attributes_for :uploaded_document
	belongs_to :documentable, polymorphic: true
end

show_registration.html.erb

<p id="notice"><%= notice %></p>

<p>
  <strong>Course:</strong>
  <%= @class_registration.classdate.course.name %>
</p>

<p>
  <strong>Person:</strong>
  <%= @class_registration.person.full_name %>
</p>

<p>
  <strong>Status:</strong>
  <%= @class_registration.registration_status.join("<br>").html_safe %>
</p>

<%= form_for @class_registration do |f| %>
  <%= f.fields_for :document_assignments, @class_registration.document_assignments.build do |ff| %>
    <% @class_registration.missing_documents.each do |doc| %>
      <%= ff.label doc.name %>
      <%= ff.collection_select(:uploaded_document_id, @class_registration.person.uploaded_documents, :id, :name) %>
      <%= ff.fields_for @class_registration.uploaded_documents.build do |fff| %>
        Upload a new form: <%= fff.file_field :document %>
        <%= fff.hidden_field :owner_id, :value => @class_registration.person.id %>
        <%= fff.hidden_field :owner_type, :value => "Person" %>
      <% end %>
    <% end %>
  <% end %>
  <%= f.submit %>
<% end %>

<%= link_to 'Edit', edit_class_registration_path(@class_registration) %> |
<%= link_to 'Back', class_registrations_path %>

uploaded_document.rb

class UploadedDocument < ActiveRecord::Base
	has_many :document_assignments
	belongs_to :owner, polymorphic: true
	has_attached_file :document
end