I'm using the awesome_nested_set plugin in my Rails project. I have two models that look like this (simplified):

class Customer < ActiveRecord::Base
  has_many :categories
end

class Category < ActiveRecord::Base
  belongs_to :customer

  # Columns in the categories table: lft, rgt and parent_id
  acts_as_nested_set :scope => :customer_id

  validates_presence_of :name
  # Further validations...
end

The tree in the database is constructed as expected. All the values of parent_id, lft and rgt are correct. The tree has multiple root nodes (which is of course allowed in awesome_nested_set).

Now, I want to render all categories of a given customer in a correctly sorted tree like structure: for example nested <ul> tags. This wouldn't be too difficult but I need it to be efficient (the less sql queries the better).

Update: Figured out that it is possible to calculate the number of children for any given Node in the tree without further SQL queries: number_of_children = (node.rgt - node.lft - 1)/2. This doesn't solve the problem but it may prove to be helpful.