Gem Awesome_nested_set has useful helper(nested_set_options) which provides buiding tree for select elements.
<%= f.select :parent_id, nested_set_options(Category, @category) {|i| "#{'-' * i.level} #{i.name}" } %>
Gem Mongoid_nested_set is a mongo port of awesome_nested_set. But it hasn't such helper. How can I solve this?