Nested selection in Rails

I'm working on a Rails app that uses categories

for items

.

My model category

is self-connected, so categories can be nested:

class Category < ActiveRecord::Base
  has_many :items

  # Self Join (categories can have subcategories)
  has_many   :subcategories, :class_name => "Category", :foreign_key => "parent_id"
  belongs_to :parent,        :class_name => "Category"
  ...
end

      

        
        
        
      

    

I have a form that allows the user to create item

that currently displays all the categories in a select element, but they are all listed together:

<%= f.label :category_id %>
<%= select :item, :category_id, Category.all.collect {|c| [ c.title, c.id ]} %>

      

        
        
        
      

    

So, the selection looks something like this:

Category1
Category2
Category3BelongsTo2
Category4BelongsTo1

      

        
        
        
      

    

But I want:

Category1
  - Category4BelongsTo1
Category2
  - Category3BelongsTo2

      

        
        
        
      

    

Is there a helper for this (which would be awesome!)? If not, how can I do this?

Thanks!