Specify this act if you want to model a tree structure by providing a parent association and a children association. This act requires that you have a foreign key column, which by default is called parent_id.

class Category < ActiveRecord::Base
  acts_as_tree :order => "name"
end

Example:
root
 \_ child1
      \_ subchild1
      \_ subchild2

root      = Category.create("name" => "root")
child1    = root.children.create("name" => "child1")
subchild1 = child1.children.create("name" => "subchild1")

root.parent   # => nil
child1.parent # => root
root.children # => [child1]
root.children.first.children.first # => subchild1

In addition to the parent and children associations, the following instance methods are added to the class after specifying the act:

  • siblings : Returns all the children of the parent, excluding the current node ([ subchild2 ] when called from subchild1)

  • self_and_siblings : Returns all the children of the parent, including the current node ([ subchild1, subchild2 ] when called from subchild1)

  • ancestors : Returns all the ancestors of the current node ([child1, root] when called from subchild2)

  • root : Returns the root of the current node (root when called from subchild2)

Methods
A
Instance Public methods
acts_as_tree(options = {})

Configuration options are:

  • foreign_key - specifies the column name to use for tracking of the tree (default: parent_id)

  • order - makes it possible to sort the children according to this SQL snippet.

  • counter_cache - keeps a count in a children_count column if set to true (default: false).

Source: show 

# File rails/activerecord/lib/active_record/acts/tree.rb, line 42
        def acts_as_tree(options = {})
          configuration = { :foreign_key => "parent_id", :order => nil, :counter_cache => nil }
          configuration.update(options) if options.is_a?(Hash)

          belongs_to :parent, :class_name => name, :foreign_key => configuration[:foreign_key], :counter_cache => configuration[:counter_cache]
          has_many :children, :class_name => name, :foreign_key => configuration[:foreign_key], :order => configuration[:order], :dependent => :destroy

          class_eval "            include ActiveRecord::Acts::Tree::InstanceMethods

            def self.roots
              find(:all, :conditions => "#{configuration[:foreign_key]} IS NULL", :order => #{configuration[:order].nil? ? "nil" : %Q{"#{configuration[:order]}"}})
            end

            def self.root
              find(:first, :conditions => "#{configuration[:foreign_key]} IS NULL", :order => #{configuration[:order].nil? ? "nil" : %Q{"#{configuration[:order]}"}})
            end
"
        end