Rails Helper to build Hierachical HTML List

helper.rb

module TreeListHelper
  # the collection need to be the root parents
  def tree_list(collection)
    content_tag(:ul) do
      collection.each do |item|
        if item.children.any?
          concat(
            content_tag(:li, id: item.id) do
              concat(item.name)
              concat(tree_list(item.children))
            end
          )
        else
          concat(content_tag(:li, item.name, id: item.id))
        end
      end
    end
  end
end

This code will generate the following structure:

 <ul>
  <li>Immigration
    <ul>
      <li>News &amp; Stories</li>
      <li>The Data</li>
    </ul>
  </li>
  <li>Student Loans
    <ul>
      <li>News &amp; Stories</li>
      <li>The Data</li>
    </ul>
  </li>
</ul>

what if the collection is not the root parent?