Ancestry is a gem/plugin that allows the records of a Ruby on Rails ActiveRecord model to be organised as a tree structure (or hierarchy). It uses a single, intuitively formatted database column, using a variation on the materialised path pattern. It exposes all the standard tree structure relations (ancestors, parent, root, children, siblings, descendants) and all of them can be fetched in a single SQL query. Additional features are STI support, scopes, depth caching, depth constraints, easy migration from older plugins/gems, integrity checking, integrity restoration, arrangement of (sub)tree into hashes and different strategies for dealing with orphaned records.

Code Quality Rank: L4
Monthly Downloads: 417,107
Programming language: Ruby
License: MIT License
Latest version: v4.2.0

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Ancestry is a gem that allows the records of a Ruby on Rails ActiveRecord model to be organised as a tree structure (or hierarchy). It employs the materialised path pattern and exposes all the standard tree structure relations (ancestors, parent, root, children, siblings, descendants), allowing all of them to be fetched in a single SQL query. Additional features include STI support, scopes, depth caching, depth constraints, easy migration from older gems, integrity checking, integrity restoration, arrangement of (sub)trees into hashes, and various strategies for dealing with orphaned records.


  • Ancestry 2.x supports Rails 4.1 and earlier
  • Ancestry 3.x supports Rails 5.0 and 4.2
  • Ancestry 4.0 only supports rails 5.0 and higher


Follow these simple steps to apply Ancestry to any ActiveRecord model:


  • Add to Gemfile:
# Gemfile

gem 'ancestry'
  • Install required gems:
$ bundle install

Add ancestry column to your table

  • Create migration:
$ rails g migration add_ancestry_to_[table] ancestry:string:index
# or use different column name of your choosing. e.g. name:
# rails g migration add_name_to_[people] name:string:index
  • Migrate your database:
$ rake db:migrate

Depending upon your comfort with databases, you may want to create the column with C or POSIX encoding. This is a more primitive encoding and just compares bytes. Since this column will just contains numbers and slashes, it works much better. It also works better for the uuid case as well.

Alternatively, if you create a text_pattern_ops index for your postgresql column, subtree selection will use an efficient index for you regardless of whether you created the column with POSIX encoding.

If you opt out of this, and are trying to run tests on postgres, you may need to set the environment variable COLLATE_SYMBOLS=false. Sorry to say that a discussion on this topic is out of scope. The important take away is postgres sort order is not consistent across operating systems but other databases do not have this same issue.

NOTE: A Btree index (as is recommended) has a limitaton of 2704 characters for the ancestry column. This means you can't have an tree with a depth that is too great (~> 900 items at most).

Add ancestry to your model

  • Add to app/models/[model.rb]:
# app/models/[model.rb]

class [Model] < ActiveRecord::Base
   has_ancestry # or alternatively as below:
   # has_ancestry ancestry_column: :name ## if you've used a different column name

Your model is now a tree!

Using acts_as_tree instead of has_ancestry

In version 1.2.0, the acts_as_tree method was renamed to has_ancestry in order to allow usage of both the acts_as_tree gem and the ancestry gem in a single application. The acts_as_tree method will continue to be supported in the future.

Organising records into a tree

You can use the parent attribute to organise your records into a tree. If you have the id of the record you want to use as a parent and don't want to fetch it, you can also use parent_id. Like any virtual model attributes, parent and parent_id can be set using parent= and parent_id= on a record or by including them in the hash passed to new, create, create!, update_attributes and update_attributes!. For example:

TreeNode.create! :name => 'Stinky', :parent => TreeNode.create!(:name => 'Squeeky').

Children can be created through the children relation on a node: node.children.create :name => 'Stinky'.

Tree Navigation

The node with the large border is the reference node (the node from which the navigation method is invoked.) The yellow nodes are those returned by the method.

parent root1 ancestors
parent root ancestors
nil for a root node self for a root node root..parent
parent_id root_id ancestor_ids
has_parent? is_root? ancestors?
parent_of? root_of? ancestor_of?
children descendants indirects
children descendants indirects
child_ids descendant_ids indirect_ids
child_of? descendant_of? indirect_of?
siblings subtree path
siblings subtree path
includes self self..indirects root..self
sibling_ids subtree_ids path_ids

1. [other root records are considered siblings]↩

has_ancestry options

The has_ancestry method supports the following options:

:ancestry_column       Pass in a symbol to store ancestry in a different column
:orphan_strategy       Instruct Ancestry what to do with children of a node that is destroyed:
                       :destroy   All children are destroyed as well (default)
                       :rootify   The children of the destroyed node become root nodes
                       :restrict  An AncestryException is raised if any children exist
                       :adopt     The orphan subtree is added to the parent of the deleted node
                                  If the deleted node is Root, then rootify the orphan subtree
:cache_depth           Cache the depth of each node in the 'ancestry_depth' column (default: false)
                       If you turn depth_caching on for an existing model:
                       - Migrate: add_column [table], :ancestry_depth, :integer, :default => 0
                       - Build cache: TreeNode.rebuild_depth_cache!
:depth_cache_column    Pass in a symbol to store depth cache in a different column
:primary_key_format    Supply a regular expression that matches the format of your primary key
                       By default, primary keys only match integers ([0-9]+)
:touch                 Instruct Ancestry to touch the ancestors of a node when it changes, to
                       invalidate nested key-based caches. (default: false)
:counter_cache         Boolean whether to create counter cache column accessor. 
                       Default column name is `children_count`. 
                       Pass symbol to use different column name (default: false)

(Named) Scopes

The navigation methods return scopes instead of records, where possible. Additional ordering, conditions, limits, etc. can be applied and the results can be retrieved, counted, or checked for existence:

node.children.where(:name => 'Mary').exists?
node.subtree.order(:name).limit(10).each { ... }

A couple of class-level named scopes are included:

roots                   Root nodes
ancestors_of(node)      Ancestors of node, node can be either a record or an id
children_of(node)       Children of node, node can be either a record or an id
descendants_of(node)    Descendants of node, node can be either a record or an id
indirects_of(node)      Indirect children of node, node can be either a record or an id
subtree_of(node)        Subtree of node, node can be either a record or an id
siblings_of(node)       Siblings of node, node can be either a record or an id

It is possible thanks to some convenient rails magic to create nodes through the children and siblings scopes:


Selecting nodes by depth

With depth caching enabled (see has_ancestry options), an additional five named scopes can be used to select nodes by depth:

before_depth(depth)     Return nodes that are less deep than depth (node.depth < depth)
to_depth(depth)         Return nodes up to a certain depth (node.depth <= depth)
at_depth(depth)         Return nodes that are at depth (node.depth == depth)
from_depth(depth)       Return nodes starting from a certain depth (node.depth >= depth)
after_depth(depth)      Return nodes that are deeper than depth (node.depth > depth)

Depth scopes are also available through calls to descendants, descendant_ids, subtree, subtree_ids, path and ancestors (with relative depth). Note that depth constraints cannot be passed to ancestor_ids or path_ids as both relations can be fetched directly from the ancestry column without needing a query. Use ancestors(depth_options).map(&:id) or ancestor_ids.slice(min_depth..max_depth) instead.

node.ancestors(:from_depth => -6, :to_depth => -4)
node.descendants(:from_depth => 2, :to_depth => 4)

STI support

To use with STI: create a STI inheritance hierarchy and build a tree from the different classes/models. All Ancestry relations that were described above will return nodes of any model type. If you do only want nodes of a specific subclass, a type condition is required.


A subtree can be arranged into nested hashes for easy navigation after database retrieval. TreeNode.arrange could, for instance, return:

  #<TreeNode id: 100018, name: "Stinky", ancestry: nil> => {
    #<TreeNode id: 100019, name: "Crunchy", ancestry: "100018"> => {
      #<TreeNode id: 100020, name: "Squeeky", ancestry: "100018/100019"> => {}
    #<TreeNode id: 100021, name: "Squishy", ancestry: "100018"> => {}

The arrange method can work on a scoped class (TreeNode.find_by(:name => 'Crunchy').subtree.arrange), and can take ActiveRecord find options. If you want ordered hashes, pass the order to the method instead of the scope as follows:

TreeNode.find_by(:name => 'Crunchy').subtree.arrange(:order => :name).

The arrange_serializable method returns the arranged nodes as a nested array of hashes. Order can be passed in the same fashion as to the arrange method: TreeNode.arrange_serializable(:order => :name) The result can easily be serialized to json with to_json or other formats. You can also supply your own serialization logic with blocks.

Using ActiveModel serializers:

TreeNode.arrange_serializable { |parent, children| MySerializer.new(parent, children: children) }.

Or plain hashes:

TreeNode.arrange_serializable do |parent, children|
     my_id: parent.id,
     my_children: children


The sort_by_ancestry class method: TreeNode.sort_by_ancestry(array_of_nodes) can be used to sort an array of nodes as if traversing in preorder. (Note that since materialised path trees do not support ordering within a rank, the order of siblings is dependant upon their original array order.)

Migrating from plugin that uses parent_id column

Most current tree plugins use a parent_id column (has_ancestry, awesome_nested_set, better_nested_set, acts_as_nested_set). With Ancestry it is easy to migrate from any of these plugins. To do so, use the build_ancestry_from_parent_ids! method on your ancestry model.


  1. Add ancestry column to your table

    • Create migration: rails g migration [add_ancestry_to_](table) ancestry:string
    • Add index to migration: add_index [table], :ancestry (UP) / remove_index [table], :ancestry (DOWN)
    • Migrate your database: rake db:migrate
  2. Remove old tree gem and add in Ancestry to Gemfile

    • See 'Installation' for more info on installing and configuring gems
  3. Change your model

    • Remove any macros required by old plugin/gem from [app/models/](model).rb
    • Add to [app/models/](model).rb: has_ancestry
  4. Generate ancestry columns

    • In rails console: [model].build_ancestry_from_parent_ids!
    • Make sure it worked ok: [model].check_ancestry_integrity!
  5. Change your code

    • Most tree calls will probably work fine with ancestry
    • Others must be changed or proxied
    • Check if all your data is intact and all tests pass
  6. Drop parent_id column:

    • Create migration: rails g migration [remove_parent_id_from_](table)
    • Add to migration: remove_column [table], :parent_id
    • Migrate your database: rake db:migrate

Running Tests

git clone [email protected]:stefankroes/ancestry.git
cd ancestry
cp test/database.example.yml test/database.yml
appraisal install
# all tests
appraisal rake test
# single test version (sqlite and rails 5.0)
appraisal sqlite3-ar-50 rake test


Ancestry stores a path from the root to the parent for every node. This is a variation on the materialised path database pattern. It allows Ancestry to fetch any relation (siblings, descendants, etc.) in a single SQL query without the complicated algorithms and incomprehensibility associated with left and right values. Additionally, any inserts, deletes and updates only affect nodes within the affected node's own subtree.

In the example above, the ancestry column is created as a string. This puts a limitation on the depth of the tree of about 40 or 50 levels. To increase the maximum depth of the tree, increase the size of the string or use text to remove the limitation entirely. Changing it to a text will however decrease performance because an index cannot be put on the column in that case.

The materialised path pattern requires Ancestry to use a 'like' condition in order to fetch descendants. The wild character (%) is on the right of the query, so indexes should be used.

Contributing and license

Question? Bug report? Faulty/incomplete documentation? Feature request? Please post an issue on 'http://github.com/stefankroes/ancestry/issues'. Make sure you have read the documentation and you have included tests and documentation with any pull request.

Copyright (c) 2016 Stefan Kroes, released under the MIT license

*Note that all licence references and agreements mentioned in the Ancestry README section above are relevant to that project's source code only.