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 uses a single database column, employing the materialised path pattern. It exposes all the standard tree structure relations (ancestors, parent, root, children, siblings, descendants) and allows all of them to be fetched in a single SQL query. Additional features are 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.

NOTE:

• 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

To apply Ancestry to any ActiveRecord model, follow these simple steps:

• Add to Gemfile:

# Gemfile

gem 'ancestry'

• Install required gems:

$ bundle install

• Create migration:

$ rails g migration add_ancestry_to_[table] ancestry:string:index

• Migrate your database:

$ rake db:migrate

• Add to app/models/[model.rb]:

# app/models/[model.rb]

class [Model] < ActiveRecord::Base
   has_ancestry
end

Your model is now a tree!

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.

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')

You can also create children through the children relation on a node:

node.children.create :name => 'Stinky'

To navigate an Ancestry model, use the following instance methods:

* If the record is a root, other root records are considered siblings * Siblings returns the record itself

There are also instance methods to determine the relationship between 2 nodes:

In all examples the node with the large border is the reference node, the node from which the navigation method is invoked. The yellow nodes are the nodes returned by the method.

parent	root	ancestors
path	children	siblings
descendants	indirects	subtree

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)

Where possible, the navigation methods return scopes instead of records. This means additional ordering, conditions, limits, etc. can be applied and that the result can be either retrieved, counted, or checked for existence. For example:

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

For convenience, a couple of named scopes are included at the class level:

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

Thanks to some convenient rails magic, it is even possible to create nodes through the children and siblings scopes:

node.children.create
node.siblings.create!
TestNode.children_of(node_id).new
TestNode.siblings_of(node_id).create

When depth caching is enabled (see has_ancestry options), five more named scopes can be used to select nodes on their 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)

The depth scopes are also available through calls to descendants, descendant_ids, subtree, subtree_ids, path and ancestors. In this case, depth values are interpreted relatively. Some examples:

node.subtree(:to_depth => 2)      Subtree of node, to a depth of node.depth + 2 (self, children and grandchildren)
node.subtree.to_depth(5)          Subtree of node to an absolute depth of 5
node.descendants(:at_depth => 2)  Descendant of node, at depth node.depth + 2 (grandchildren)
node.descendants.at_depth(10)     Descendants of node at an absolute depth of 10
node.ancestors.to_depth(3)        The oldest 4 ancestors of node (its root and 3 more)
node.path(:from_depth => -2)      The node's grandparent, parent and the node itself

node.ancestors(:from_depth => -6, :to_depth => -4)
node.path.from_depth(3).to_depth(4)
node.descendants(:from_depth => 2, :to_depth => 4)
node.subtree.from_depth(10).to_depth(12)

Please note that depth constraints cannot be passed to ancestor_ids and path_ids. The reason for this is that both these relations can be fetched directly from the ancestry column without performing a database query. It would require an entirely different method of applying the depth constraints which isn't worth the effort of implementing. You can use ancestors(depth_options).map(&:id) or ancestor_ids.slice(min_depth..max_depth) instead.

Ancestry works fine with STI. Just create a STI inheritance hierarchy and build an Ancestry 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 you'll have to add a condition on type for that.

Ancestry can arrange an entire subtree into nested hashes for easy navigation after retrieval from the database. TreeNode.arrange could for example 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 also works on a scoped class, for example:

TreeNode.find_by_name('Crunchy').subtree.arrange

The arrange method takes ActiveRecord find options. If you want your hashes to be ordered, you should pass the order to the arrange method instead of to the scope. example:

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

To get the arranged nodes as a nested array of hashes for serialization:

TreeNode.arrange_serializable

[
  {
    "ancestry" => nil, "id" => 1, "children" => [
      { "ancestry" => "1", "id" => 2, "children" => [] }
    ]
  }
]

You can also supply your own serialization logic using blocks:

For example, using ActiveModel Serializers:

TreeNode.arrange_serializable do |parent, children|
  MySerializer.new(parent, children: children)
end

Or plain hashes:

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

The result of arrange_serializable can easily be serialized to json with to_json, or some other format:

TreeNode.arrange_serializable.to_json

You can also pass the order to the arrange_serializable method just as you can pass it to the arrange method:

TreeNode.arrange_serializable(:order => :name)

If you just want to sort an array of nodes as if you were traversing them in preorder, you can use the sort_by_ancestry class method:

TreeNode.sort_by_ancestry(array_of_nodes)

Note that since materialised path trees don't support ordering within a rank, the order of siblings depends on their order in the original array.

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. These steps provide a more detailed explanation:

1. Add ancestry column to your table

   • Create migration: rails g migration add_ancestry_to_ 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

I don't see any way Ancestry tree integrity could get compromised without explicitly setting cyclic parents or invalid ancestry and circumventing validation with update_attribute. If you do, please let me know.

Ancestry includes some methods for detecting integrity problems and restoring integrity just to be sure. To check integrity, use: [Model].check_ancestry_integrity!. An AncestryIntegrityException will be raised if there are any problems. You can also specify :report => :list to return an array of exceptions or :report => :echo to echo any error messages. To restore integrity use: [Model].restore_ancestry_integrity!.

For example, from IRB:

>> stinky = TreeNode.create :name => 'Stinky'
$  #<TreeNode id: 1, name: "Stinky", ancestry: nil>
>> squeeky = TreeNode.create :name => 'Squeeky', :parent => stinky
$  #<TreeNode id: 2, name: "Squeeky", ancestry: "1">
>> stinky.update_attribute :parent, squeeky
$  true
>> TreeNode.all
$  [#<TreeNode id: 1, name: "Stinky", ancestry: "1/2">, #<TreeNode id: 2, name: "Squeeky", ancestry: "1/2/1">]
>> TreeNode.check_ancestry_integrity!
!! Ancestry::AncestryIntegrityException: Conflicting parent id in node 1: 2 for node 1, expecting nil
>> TreeNode.restore_ancestry_integrity!
$  [#<TreeNode id: 1, name: "Stinky", ancestry: 2>, #<TreeNode id: 2, name: "Squeeky", ancestry: nil>]

Additionally, if you think something is wrong with your depth cache:

>> TreeNode.rebuild_depth_cache!

git clone git@github.com:stefankroes/ancestry.git
cd ancestry
cp test/database.example.yml test/database.yml
bundle
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.

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