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

what if the collection is not the root parent?