Easily and efficiently make your ActiveRecord model support hierarchies
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Closure Tree

Closure_tree lets your ActiveRecord models act as nodes in a tree data structure

Common applications include modeling hierarchical data, like tags, page graphs in CMSes, and tracking user referrals.

Build Status Gem Version

Substantially more efficient than ancestry and acts_as_tree, and even more awesome than awesome_nested_set, closure_tree has some great features:

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

Table of Contents


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). There are a number of options you can pass in, too.

  4. Add a migration to add a parent_id column to the model you want to act_as_tree. You may want to also add a column for deterministic ordering of children, but that's optional.

    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, TagHierarchy) 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 your tag_hierarchies 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:

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

Or by appending to the children collection:

child2 = Tag.new(:name => 'Second Child')
parent.children << child2

Or by calling the "add_child" method:

child3 = Tag.new(:name => 'Third Child')
parent.add_child child3


=> ["Grandparent", "Parent", "First Child", "Second Child", "Third Child"]

=> ["Grandparent", "Parent", "First Child"]


We can do all the node creation and add_child calls 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).

Moving nodes around the tree

Nodes can be moved around to other parents, and closure_tree moves the node's descendancy to the new parent for you:

d = Tag.find_or_create_by_path %w(a b c d)
h = Tag.find_or_create_by_path %w(e f g h)
e = h.root
d.add_child(e) # "d.children << e" would work too, of course
=> ["a", "b", "c", "d", "e", "f", "g", "h"]

Nested hashes

hash_tree provides a method for rendering a subtree as an ordered nested hash:

b = Tag.find_or_create_by_path %w(a b)
a = b.parent
b2 = Tag.find_or_create_by_path %w(a b2)
d1 = b.find_or_create_by_path %w(c1 d1)
c1 = d1.parent
d2 = b.find_or_create_by_path %w(c2 d2)
c2 = d2.parent

=> {a => {b => {c1 => {d1 => {}}, c2 => {d2 => {}}}, b2 => {}}}

Tag.hash_tree(:limit_depth => 2)
=> {a => {b => {}, b2 => {}}}

=> {b => {c1 => {d1 => {}}, c2 => {d2 => {}}}}

b.hash_tree(:limit_depth => 2)
=> {b => {c1 => {}, c2 => {}}}

If your tree is large (or might become so), use :limit_depth.

Without this option, hash_tree will load the entire contents of that table into RAM. Your server may not be happy trying to do this.

HT: ancestry and elhoyos

Graph visualization

to_dot_digraph is suitable for passing into Graphviz.

For example, for the above tree, write out the DOT file with ruby:

File.open("example.dot", "w") { |f| f.write(Tag.root.to_dot_digraph) }

Then, in a shell, dot -Tpng example.dot > example.png, which produces:

Example tree

If you want to customize the label value, override the #to_digraph_label instance method in your model.

Just for kicks, this is the test tree I used for proving that preordered tree traversal was correct:

Preordered test tree

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 class name. This defaults to the singular name of the model + "Hierarchy", like TagHierarchy.
  • :hierarchy_table_name to override the hierarchy table name. This defaults to the singular name of the model + "_hierarchies", like tag_hierarchies.
  • :dependent determines what happens when a node is destroyed. Defaults to nullify.
    • :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").
  • :order used to set up deterministic ordering

Accessing Data

Class methods

  • Tag.root returns an arbitrary root node
  • Tag.roots returns all root nodes
  • Tag.leaves returns all leaf nodes
  • Tag.hash_tree returns an ordered, nested hash that can be depth-limited.
  • Tag.find_by_path(path) returns the node whose name path is path. See (#find_or_create_by_path).
  • Tag.find_or_create_by_path(path) returns the node whose name path is path, and will create the node if it doesn't exist already.See (#find_or_create_by_path).
  • Tag.find_all_by_generation(generation_level) returns the descendant nodes who are generation_level away from a root. Tag.find_all_by_generation(0) is equivalent to Tag.roots.

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 is scoped to all leaf nodes in self_and_descendants.
  • tag.depth returns the depth, or "generation", for this node in the tree. A root node will have a value of 0.
  • tag.parent returns the node's immediate parent. Root nodes will return nil.
  • tag.children is a has_many of immediate children (just those nodes whose parent is the current node).
  • tag.child_ids is an array of the IDs of the children.
  • tag.ancestors is a ordered scope of [ parent, grandparent, great grandparent, … ]. Note that the size of this array will always equal tag.depth.
  • tag.ancestor_ids is an array of the IDs of the ancestors.
  • tag.self_and_ancestors returns a scope containing self, parent, grandparent, great grandparent, etc.
  • tag.siblings returns a scope containing all nodes with the same parent as tag, excluding self.
  • tag.sibling_ids returns an array of the IDs of the siblings.
  • tag.self_and_siblings returns a scope containing all nodes with the same parent as tag, including self.
  • tag.descendants returns a scope of all children, childrens' children, etc., excluding self ordered by depth.
  • tag.descendant_ids returns an array of the IDs of the descendants.
  • tag.self_and_descendants returns a scope of all children, childrens' children, etc., including self, ordered by depth.
  • tag.hash_tree returns an ordered, nested hash that can be depth-limited.
  • tag.find_by_path(path) returns the node whose name path from tag is path. See (#find_or_create_by_path).
  • tag.find_or_create_by_path(path) returns the node whose name path from tag is path, and will create the node if it doesn't exist already.See (#find_or_create_by_path).
  • tag.find_all_by_generation(generation_level) returns the descendant nodes who are generation_level away from tag.
    • tag.find_all_by_generation(0).to_a == [tag]
    • tag.find_all_by_generation(1) == tag.children
    • tag.find_all_by_generation(2) will return the tag's grandchildren, and so on.
  • 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 with STI

Polymorphic models using single table inheritance (STI) 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

Please note that Rails (<= 3.2) doesn't handle polymorphic associations correctly if you use the :type attribute, so this doesn't work:

# BAD: ActiveRecord ignores the :type attribute:
root.children.create(:name => "child", :type => "WhenTag")

Instead, use either .add_child or children <<:

a = Tag.create!(:name => "a")
b = WhenTag.new(:name => "b")
a.children << b
c = WhatTag.new(:name => "c")

See issue 43 for more information.

Deterministic ordering

By default, children will be ordered by your database engine, which may not be what you want.

If you want to order children alphabetically, and your model has a name column, you'd do this:

class Tag < ActiveRecord::Base
  acts_as_tree :order => 'name'

If you want a specific order, add a new integer column to your model in a migration:

t.integer :sort_order

and in your model:

class OrderedTag < ActiveRecord::Base
  acts_as_tree :order => 'sort_order'

When you enable order, you'll also have the following new methods injected into your model:

  • tag.siblings_before is a scope containing all nodes with the same parent as tag, whose sort order column is less than self. These will be ordered properly, so the last element in scope will be the sibling immediately before self
  • tag.siblings_after is a scope containing all nodes with the same parent as tag, whose sort order column is more than self. These will be ordered properly, so the first element in scope will be the sibling immediately "after" self

If your order column is an integer attribute, you'll also have these:

  • The class method #roots_and_descendants_preordered, which returns all nodes in your tree, pre-ordered.

  • node1.self_and_descendants_preordered which will return descendants, pre-ordered.

  • node1.prepend_sibling(node2) which will

    1. set node2 to the same parent as node1,
    2. set node2's order column to 1 less than node1's value, and
    3. decrement the order_column of all children of node1's parents whose order_column is <>>= node2's new value by 1.
  • node1.append_sibling(node2) which will

    1. set node2 to the same parent as node1,
    2. set node2's order column to 1 more than node1's value, and
    3. increment the order_column of all children of node1's parents whose order_column is >= node2's new value by 1.
root = OrderedTag.create(:name => "root")
a = OrderedTag.create(:name => "a", :parent => "root")
b = OrderedTag.create(:name => "b")
c = OrderedTag.create(:name => "c")

# We have to call 'root.reload.children' because root won't be in sync with the database otherwise:

=> ["a", "b"]

=> ["b", "a"]

=> ["b", "a", "c"]

=> ["b", "c", "a"]


Several methods, especially #rebuild and #find_or_create_by_path, cannot run concurrently correctly. #find_or_create_by_path, for example, may create duplicate nodes.

Database row-level locks work correctly with PostgreSQL, but MySQL's row-level locking is broken, and erroneously reports deadlocks where there are none. To work around this, and have a consistent implementation for both MySQL and PostgreSQL, with_advisory_lock is used automatically to ensure correctness.

If you are already managing concurrency elsewhere in your application, and want to disable the use of with_advisory_lock, pass :with_advisory_lock => false in the options hash:

class Tag
  acts_as_tree :with_advisory_lock => false

Note that you will eventually have data corruption if you disable advisory locks, write to your database with multiple threads, and don't provide an alternative mutex.


Does this gem support multiple parents?

No. This gem's API is based on the assumption that each node has either 0 or 1 parent.

The underlying closure tree structure will support multiple parents, but there would be many breaking-API changes to support it. I'm open to suggestions and pull requests.

How do I use this with test fixtures?

Test fixtures aren't going to be running your after_save hooks after inserting all your fixture data, so you need to call .rebuild! before your test runs. There's an example in the spec tag_spec.rb:

  describe "Tag with fixtures" do
    fixtures :tags
    before :each do
      Tag.rebuild! # <- required if you use fixtures

However, if you're just starting with Rails, may I humbly suggest you adopt a factory library, rather than using fixtures? Lots of people have written about this already.


Closure tree is tested under every valid combination of

  • Ruby 1.8.7, Ruby 1.9.3, and Ruby 2.0.0
  • The latest Rails 3.0, 3.1, 3.2, and 4.0 branches, and
  • MySQL and PostgreSQL. SQLite works in a single-threaded environment.

Assuming you're using rbenv, you can use tests.sh to run the test matrix locally.

Parallelism is not tested with Rails 3.0.x nor 3.1.x due to this known issue.

Change log


  • Added support for Rails 4.0.0.rc1 and Ruby 2.0.0 (while maintaining backward compatibility with Rails 3, BOOYA)
  • Added #to_dot_digraph, suitable for Graphviz rendering


  • Numeric, deterministically ordered siblings will always be [0..#{self_and_siblings.count}] (previously, the sort order might use negative values, which broke the preordering). Resolves issue 49. Thanks for the help, Leonel Galan, Juan Hoyos, and Michael Elfassy!

  • The order option can be a symbol now. Resolves issue 46.


  • Moved all of closure_tree's implementation-detail methods into a ClosureTree::Support instance, which removes almost all of the namespace pollution in your models that wasn't for normal consumption. If you were using any of these methods, they're now available through the "_ct" class and instance member.

    This change may break consumers, so I incremented the major version number, even though no new functionality was released.


  • Prevent faulty SQL statement when #siblings is called on an unsaved records. Resolves issue 52. Perfect pull request by Gary Greyling.

  • The .roots class method now correctly respects the :order option. Resolves issue 53. Thanks for finding this, Brendon Muir!


  • Multipart constant names like "Admin::PageHierarchy" are now supported. Resolves issue 47. Thanks for the perfect pull request, Simon Menke!

  • Committing transactions involving large numbers of hierarchy model classes was very slow due to hash collisions in the hierarchy class. A better hash implementation addressed issue 48. Thanks, Joel Turkel!


  • Added #roots_and_descendants_preordered. Thanks for the suggestion, Leonel Galan!


  • Added .child_ids.
  • Removed dependent => destroy on the descendant_hierarchy and ancestor_hierarchy collections (they were a mistake).
  • Clarified documentation for creation and child associations. Because Tag.create!(:parent => ...) requires a .reload, I removed it as an example.

All three of these improvements were suggested by Andrew Bromwich. Thanks!


  • find_by_path uses 1 SELECT now. BOOM.


  • Double-check locking for find_or_create_by_path


  • Support for preordered descendants. This requires a numeric sort order column. Resolves feature request 38.
  • Moved modules from acts_as_tree into separate files


Due to MySQL's inability to lock rows properly, I've switched to advisory_locks for all write paths. This will prevent deadlocks, addressing issue 41.


  • Support for UUID primary keys. Addresses issue 40. Thanks for the pull request, Julien!


  • Moved requires into ActiveSupport.on_load
  • Added require 'with_advisory_lock'


Thread safety!

  • Advisory locks were integrated with the class-level find_or_create_by_path and rebuild!.
  • Pessimistic locking is used by the instance-level find_or_create_by_path.


  • Don Morrison massaged the #hash_tree query to be more efficient, and found a bug in hash_tree's query that resulted in duplicate rows, wasting time on the ruby side.


  • Added workaround for ActiveRecord::Observer usage pre-db-creation. Addresses issue 32. Thanks, Don Morrison!






  • Fixed issue 23, which added support for #siblings when sort_order wasn't specified. Thanks, Gary Greyling!


  • Fixed issue 20, which affected deterministic ordering when siblings where different STI classes. Thanks, edwinramirez!


Added support for:

  • :hierarchy_class_name as an option
  • ActiveRecord::Base.table_name_prefix
  • ActiveRecord::Base.table_name_suffix

This addresses issue 21. Thanks, Judd Blair!



  • Fixed issue 18, which affected append_node/prepend_node ordering when the first node didn't have an explicit order_by value


  • Reverted .gemspec mistake that changed add_development_dependency to add_runtime_dependency


Fixed issue 15:

  • "parent" is now attr_accessible, which adds support for constructor-provided parents.
  • updated readme accordingly


  • Merged calebphillips' patch for a more efficient leaves query


  • Added support for partially-unsaved hierarchies issue 13:
a = Tag.new(name: "a")
b = Tag.new(name: "b")
a.children << b



  • Added ancestor_ids, descendant_ids, and sibling_ids
  • Added example spec to solve issue 9


  • Added support for deterministic ordering of nodes.


  • Switched to using has_many :though rather than has_and_belongs_to_many



  • 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!)


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


  • Support 3.2.0's fickle deprecation of InstanceMethods (Thanks, jheiss)!


  • 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


  • 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

Thanks to