Awesome Nested Set is an implementation of the nested set pattern for ActiveRecord models. It is a replacement for acts_as_nested_set and BetterNestedSet, but more awesome.

Version 2 supports Rails 3. Gem versions prior to 2.0 support Rails 2.

This is a new implementation of nested set based off of BetterNestedSet that fixes some bugs, removes tons of duplication, adds a few useful methods, and adds STI support.

Add to your Gemfile:

gem 'awesome_nested_set'

To make use of awesome_nested_set, your model needs to have 3 fields: lft, rgt, and parent_id. The names of these fields are configurable. You can also have an optional field, depth:

class CreateCategories < ActiveRecord::Migration
  def self.up
    create_table :categories do |t|
      t.string :name
      t.integer :parent_id
      t.integer :lft
      t.integer :rgt
      t.integer :depth # this is optional.
    end
  end

  def self.down
    drop_table :categories
  end
end

Enable the nested set functionality by declaring acts_as_nested_set on your model

class Category < ActiveRecord::Base
  acts_as_nested_set
end

Run rake rdoc to generate the API docs and see CollectiveIdea::Acts::NestedSet for more information.

It is highly recommended that you add an index to the rgt column on your models. Every insertion requires finding the next rgt value to use and this can be slow for large tables without an index. It is probably best to index the other fields as well (parent_id, lft, depth).

There are three callbacks called when moving a node: before_move, after_move and around_move.

class Category < ActiveRecord::Base
  acts_as_nested_set

  after_move :rebuild_slug
  around_move :da_fancy_things_around

  private

  def rebuild_slug
    # do whatever
  end

  def da_fancy_things_around
    # do something...
    yield # actually moves
    # do something else...
  end
end

Beside this there are also hooks to act on the newly added or removed children.

class Category < ActiveRecord::Base
  acts_as_nested_set  :before_add     => :do_before_add_stuff,
                      :after_add      => :do_after_add_stuff,
                      :before_remove  => :do_before_remove_stuff,
                      :after_remove   => :do_after_remove_stuff

  private

  def do_before_add_stuff(child_node)
    # do whatever with the child
  end

  def do_after_add_stuff(child_node)
    # do whatever with the child
  end

  def do_before_remove_stuff(child_node)
    # do whatever with the child
  end

  def do_after_remove_stuff(child_node)
    # do whatever with the child
  end
end

It's generally best to "whitelist" the attributes that can be used in mass assignment:

class Category < ActiveRecord::Base
  acts_as_nested_set
  attr_accessible :name, :parent_id
end

If for some reason that is not possible, you will probably want to protect the lft and rgt attributes:

class Category < ActiveRecord::Base
  acts_as_nested_set
  attr_protected :lft, :rgt
end

Coming from acts_as_tree or another system where you only have a parent_id? No problem. Simply add the lft & rgt fields as above, and then run:

Category.rebuild!

Your tree will be converted to a valid nested set. Awesome!

The view helper is called #nested_set_options.

Example usage:

<%= f.select :parent_id, nested_set_options(Category, @category) {|i| "#{'-' * i.level} #{i.name}" } %>

<%= select_tag 'parent_id', options_for_select(nested_set_options(Category) {|i| "#{'-' * i.level} #{i.name}" } ) %>

See CollectiveIdea::Acts::NestedSet::Helper for more information about the helpers.

You can learn more about nested sets at: http://threebit.net/tutorials/nestedset/tutorial1.html

Please see the 'Contributing' document.

Copyright © 2008 - 2013 Collective Idea, released under the MIT license