Implements acts_as_tree using the high performance closure tree algorithm, with support for polymorphism and "find_or_create_by_path"
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.

Closure Tree

Build Status

Closure Tree is a mostly-API-compatible replacement for the acts_as_tree and awesome_nested_set gems, but with much better mutation performance thanks to the Closure Tree storage algorithm, as well as support for polymorphism within the hierarchy.

See Bill Karwin's excellent Models for hierarchical data presentation for a description of different tree storage algorithms.


Note that closure_tree only supports Rails 3.0 and later, and has test coverage for MySQL, PostgreSQL, and SQLite.

  1. Add this to your Gemfile: gem 'closure_tree'

  2. Run bundle install

  3. Add acts_as_tree to your hierarchical model(s) (see the Available options section below for details).

  4. Add a migration to add a parent_id column to the model you want to act_as_tree.

    class AddParentIdToTag < ActiveRecord::Migration
      def change
        add_column :tag, :parent_id, :integer

    Note that if the column is null, the tag will be considered a root node.

  5. Add a database migration to store the hierarchy for your model. By default the table name will be the model's table name, followed by "_hierarchies". Note that by calling acts_as_tree, a "virtual model" (in this case, TagsHierarchy) will be added automatically, so you don't need to create it.

    class CreateTagHierarchies < ActiveRecord::Migration
      def change
        create_table :tag_hierarchies, :id => false do |t|
          t.integer  :ancestor_id, :null => false   # ID of the parent/grandparent/great-grandparent/... tag
          t.integer  :descendant_id, :null => false # ID of the target tag
          t.integer  :generations, :null => false   # Number of generations between the ancestor and the descendant. Parent/child = 1, for example.
        # For "all progeny of…" selects:
        add_index :tag_hierarchies, [:ancestor_id, :descendant_id], :unique => true
        # For "all ancestors of…" selects
        add_index :tag_hierarchies, [:descendant_id]
  6. Run rake db:migrate

  7. If you're migrating from another system where your model already has a parent_id column, run Tag.rebuild! and the …_hierarchy table will be truncated and rebuilt.

    If you're starting from scratch you don't need to call rebuild!.



Create a root node:

grandparent = Tag.create(:name => 'Grandparent')

Child nodes are created by appending to the children collection:

child = parent.children.create(:name => 'Child')

You can also append to the children collection:

child = Tag.create(:name => 'Child')
parent.children << child

Or call the "add_child" method:

parent = Tag.create(:name => 'Parent')
grandparent.add_child parent


puts grandparent.self_and_descendants.collect{ |t| }.join(" > ")
"grandparent > parent > child"

["grandparent", "parent", "child"]


We can do all the node creation and add_child calls from the prior section with one method call:

child = Tag.find_or_create_by_path(["grandparent", "parent", "child"])

You can find as well as find_or_create by "ancestry paths". Ancestry paths may be built using any column in your model. The default column is name, which can be changed with the :name_column option provided to acts_as_tree.

Note that any other AR fields can be set with the second, optional attributes argument.

child = Tag.find_or_create_by_path(%w{home chuck Photos"}, {:tag_type => "File"})

This will pass the attribute hash of {:name => "home", :tag_type => "File"} to Tag.find_or_create_by_name if the root directory doesn't exist (and {:name => "chuck", :tag_type => "File"} if the second-level tag doesn't exist, and so on).

Available options

When you include acts_as_tree in your model, you can provide a hash to override the following defaults:

  • :parent_column_name to override the column name of the parent foreign key in the model's table. This defaults to "parent_id".
  • :hierarchy_table_name to override the hierarchy table name. This defaults to the singular name of the model + "_hierarchies".
  • :dependent determines what happens when a node is destroyed. Defaults to nil.
    • :nullify will simply set the parent column to null. Each child node will be considered a "root" node. This is the default.
    • :delete_all will delete all descendant nodes (which circumvents the destroy hooks)
    • :destroy will destroy all descendant nodes (which runs the destroy hooks on each child node)
  • :name_column used by #find_or_create_by_path, #find_by_path, and ancestry_path instance methods. This is primarily useful if the model only has one required field (like a "tag").

Accessing Data

Class methods

  • Tag.root returns an arbitrary root node
  • Tag.roots returns all root nodes
  • Tag.leaves returns all leaf nodes

Instance methods

  • tag.root returns the root for this node
  • tag.root? returns true if this is a root node
  • tag.child? returns true if this is a child node. It has a parent.
  • tag.leaf? returns true if this is a leaf node. It has no children.
  • tag.leaves returns an array of all the nodes in self_and_descendants that are leaves.
  • tag.level returns the level, or "generation", for this node in the tree. A root node == 0.
  • tag.parent returns the node's immediate parent. Root nodes will return nil.
  • tag.children returns an array of immediate children (just those nodes whose parent is the current node).
  • tag.ancestors returns an array of [ parent, grandparent, great grandparent, … ]. Note that the size of this array will always equal tag.level.
  • tag.self_and_ancestors returns an array of self, parent, grandparent, great grandparent, etc.
  • tag.siblings returns an array of brothers and sisters (all at that level), excluding self.
  • tag.self_and_siblings returns an array of brothers and sisters (all at that level), including self.
  • tag.descendants returns an array of all children, childrens' children, etc., excluding self.
  • tag.self_and_descendants returns an array of all children, childrens' children, etc., including self.
  • tag.destroy will destroy a node and do something to its children, which is determined by the :dependent option passed to acts_as_tree.

Polymorphic hierarchies

Polymorphic models are supported:

  1. Create a db migration that adds a String type column to your model
  2. Subclass the model class. You only need to add acts_as_tree to your base class.
class Tag < ActiveRecord::Base
class WhenTag < Tag ; end
class WhereTag < Tag ; end
class WhatTag < Tag ; end

Change log


  • Had to increment the major version, as rebuild! will need to be called by prior consumers to support the new leaves class and instance methods.
  • Tag deletion is supported now along with :dependent => :destroy and :dependent => :delete_all
  • Switched from default rails plugin directory structure to rspec
  • Support for running specs under different database engines: export DB ; for DB in sqlite3 mysql postgresql ; do rake ; done


  • Support for polymorphic trees
  • find_by_path and find_or_create_by_path signatures changed to support constructor attributes
  • tested against Rails 3.1.3



  • Fix for ancestry-loop detection (performed by a validation, not through raising an exception in before_save)


  • Added support for ActiveRecord's whitelist_attributes (Make sure you read the Rails Security Guide, and enable config.active_record.whitelist_attributes in your config/application.rb ASAP!)

Thanks to