Rails: Nested dropdown of categories - forms

Question

I am using the ancestry gem to allow Categories to nest, and I want to be able to create a dropdown showing the nesting when a user selects a category for a new product.
I can create the nested list, but it uses the parent categories as labels for option groups instead of as selectable elements.
First attempt
<%= select_tag "product[category_id]",
option_groups_from_collection_for_select(#store.categories, :children, :name, :id, :name, #product.category.id),
{ include_blank: true } %>
Which generates something like this:
<select id="product_category_id" name="product[category_id]">
<option value=""></option>
<optgroup label="Root Category 1"></optgroup>
<optgroup label="Root Category 2">
<option value="3">Child Category 1</option>
</optgroup>
</select>
When what I really want is just:
<select id="product_category_id" name="product[category_id]">
<option value=""></option>
<option value="1">Root Category 1</option>
<option value="2">Root Category 2</option>
<optgroup>
<option value="3">Child Category 1</option>
</optgroup>
</select>
Other attempts
I also tried generating the list with content tags:
def grouped_options(categories, selected_id = nil)
body = ''
categories.map do |category|
body << content_tag(:option, category.name, { value: category.id, selected: category.id == selected_id })
if category.children.present?
body << content_tag(:optgroup) do
grouped_options(category.children)
end
end
end
body.html_safe
end
However, this approach left gaps in the list where the optgroup label should be, and you can't really go more than 1 level deep.
So...
Is there a way I can nicely nest everything in a select and be able to select each level, or should I start considering a javascript/unordered list implementation instead?