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.

Older versions: Version 3.2 supports Rails 6, 3.1 supports Rails 5 & 4. 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 optional fields: depth and children_count. These fields are configurable. Note that the children_count column must have null: false and default: 0 to function properly.

class CreateCategories < ActiveRecord::Migration
  def change
    create_table :categories do |t|
      t.string :name
      t.integer :parent_id, null: true, index: true
      t.integer :lft, null: false, index: true
      t.integer :rgt, null: false, index: true

      # optional fields
      t.integer :depth, null: false, default: 0
      t.integer :children_count, null: false, default: 0
      t.timestamps
    end
  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.

You can pass various options to acts_as_nested_set macro. Configuration options are:

• parent_column: specifies the column name to use for keeping the position integer (default: parent_id)
• primary_column: specifies the column name to use as the inverse of the parent column (default: id)
• left_column: column name for left boundary data (default: lft)
• right_column: column name for right boundary data (default: rgt)
• depth_column: column name for the depth data default (default: depth)
• scope: restricts what is to be considered a list. Given a symbol, it'll attach _id (if it hasn't been already) and use that as the foreign key restriction. You can also pass an array to scope by multiple attributes. Example: acts_as_nested_set :scope => [:notable_id, :notable_type]
• dependent: behavior for cascading destroy. If set to :destroy, all the child objects are destroyed alongside this object by calling their destroy method. If set to :delete_all (default), all the child objects are deleted without calling their destroy method. If set to :nullify, all child objects will become orphaned and become roots themselves.
• counter_cache: adds a counter cache for the number of children. defaults to false. Example: acts_as_nested_set :counter_cache => :children_count
• order_column: on which column to do sorting, by default it is the left_column_name. Example: acts_as_nested_set :order_column => :position
• touch: If set to true, then the updated_at timestamp on the ancestors will be set to the current time whenever this object is saved or destroyed (default: false)

See CollectiveIdea::Acts::NestedSet::Model::ClassMethods for a list of class methods and CollectiveIdea::Acts::NestedSet::Model for a list of instance methods added to acts_as_nested_set models

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

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 optional fields, depth and children_count.

Create a migration to add fields:

class AddNestedToCategories < ActiveRecord::Migration

  def self.up
    add_column :categories, :parent_id, :integer # Comment this line if your project already has this column
    # Category.where(parent_id: 0).update_all(parent_id: nil) # Uncomment this line if your project already has :parent_id
    add_column :categories, :lft,       :integer
    add_column :categories, :rgt,       :integer

    # optional fields
    add_column :categories, :depth,          :integer
    add_column :categories, :children_count, :integer

    # This is necessary to update :lft and :rgt columns
    Category.reset_column_information
    Category.rebuild!
  end

  def self.down
    remove_column :categories, :parent_id
    remove_column :categories, :lft
    remove_column :categories, :rgt

    # optional fields
    remove_column :categories, :depth
    remove_column :categories, :children_count
  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

Your project is now ready to run with the awesome_nested_set gem!

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!

Note: You can use Category.rebuild!(false) to skip model validations when performing the rebuild.

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.

Please see the 'Contributing' document.

Copyright © 2008–2015 Collective Idea, released under the MIT license.