Ruby on Rails
ActsAsTree

acts_as_tree allows you to traverse a hierarchical structure generated by a self referencing table.

acts_as_tree has been moved to a plugin in Rails 2.0, so it must be installed with:

script/plugin install acts_as_tree

• if you want to get the whole tree loaded at once, you probably want ActsAsNestedSet instead.
• Acts_as_threaded
• CategoryTreeUsingActsAsTree

API Documentation for acts_as_tree

Database Schema

create table tree_items (
  id int(11) unsigned not null auto_increment,
  parent_id int(11) unsigned,
  name varchar(20),
  other_content varchar(255),
  primary key (id)
);

Code

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

//top level tree item
top = TreeItem.find_by_parent_id(nil)
top.children.each do |child|
  puts child.parent.name + " is my parent. I am " + child.name
end

From code documentation:
The following accessor methods are available with the acts_as_tree modifier:

• children() – all immediate children of current object
  
• parent() – first ancestor of the current object
  
• siblings() – all children of my parent excluding me
  
• self_and_siblings() all children of my parent including me
  
• ancestors() all parent, grandparent, etc… object up to root
  
• root() the base object we descended from in the tree
  

The following options can be applied to the acts_as_tree modifier:

• foreign_key
  
• order
  
• counter_cache