A Nested Set is similar to a tree from ActsAsTree. However the nested set allows building of the entire hierarchy at once instead of querying each nodes children, and their children. When destroying an object, a before_destroy trigger prunes the rest of the branch of object under the current object.

Table Schema

create table nested_objects (
  id int(11) unsigned not null auto_increment,
  parent_id int(11),
  lft int(11),
  rgt int(11),
  name varchar(32),
  primary key (id)
);

Code

class NestedObject < ActiveRecord::Base
  acts_as_nested_set
end

Methods introduced by this feature are:

• root?() – Is the current object a root object (has no parent)?
  
• child?() – Is the current object a child object (has a parent)?
  
• unknown?() – Is the current object neither a root or child object? (not sure how you would get this)
  
• add_child(child_object) – Add an object into the structure, if the object is already a root object this will fail
  
• children_count() – Count of all descendants (not just immediate children)
  
• full_set() – Retrieve everything in one call
  
• all_children() – Retrieve everything under the current object (all descendants)
  
• direct_children() – Retrieve the current object’s immediate children
  

API documentation

Class methods
Instance methods

Alternatives

BetterNestedSet is an updated version with some additional features, available as a plugin.

Future

DHH has said that acts_as_nested_set will be relegated to a plugin in RoR 1.3.

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Question: Can a scope be provided, such that multiple trees can exist within the same database table? Importantly, this could dramatically reduce the number of rows modified upon node insertion (i.e. only those having the same scope value). If so, how is this done? I haven’t found any documentation to demonstrate this.

Answer: with BetterNestedSet or acts_as_threaded this is possible and runs properly. Rails 1.2 also seems to add a scope argument to acts_as_nested_set.

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To add acts_as_list methods see http://www.vaporbase.com/postings/List_methods_for_nested_sets