Ruby on Rails
Acts_as_threaded

Like ActsAsNestedSet, ActAsThreaded use the nested_set model : comparing to ActsAsTree, it’s slower to insert/delete/update (a whole set is updated) but the whole tree can be retrieved quickly from the database with a single query.

Great for creating threaded forums and other tree structures where read performance is a concern.

Ressources

The original link for Acts_as_threaded is now dead. More explanations can be found on this post (a zip too).

See also Plugins

Alternative

Bob Silva (who creates the plugin) proposed another way to do this without a plugin, using native Rails functionality . Same enhancements to acts_as_nested_set can be done adding 3 methods in your model :

Here’s the Code :

  acts_as_nested_set :scope => :root

  def before_create
    # Update the child object with its parents attrs
    unless self[:parent_id].to_i.zero?
      self[:depth] = parent[:depth].to_i + 1
      self[:root_id] = parent[:root_id].to_i
    end
  end

  def after_create
    # Update the parent root_id with its id
    if self[:parent_id].to_i.zero?
      self[:root_id] = self[:id]
      self.save
    else
      parent.add_child self
    end
  end

  def parent
    @parent ||= self.class.find(self[:parent_id])
  end

And here’s a migration :

class AddFieldsForActsAsNestedSet < ActiveRecord::Migration
  def self.up
    create_table :my_table_name, :force => true do |t|
      t.column "root_id", :integer
      t.column "parent_id", :integer
      t.column "lft", :integer
      t.column "rgt", :integer
      t.column "depth", :integer
    end
  end

  def self.down
    drop_table :my_table_name, :root_id
  end
end

See the post for possible update.

API documentation

Use ActsAsNestedSet, so :
Class methods
Instance methods

Questions