Xylem

Xylem provides a simple way to store and retrieve hierarchical data in ActiveRecord.

Xylem uses the Adjacency List approach to store hierarchical data, and use recursive CTEs to query through it.

That means that the storage strategy is simple: in order to map an ActiveRecord Model to a tree-like structure with parents and children, it's needed to add only one column to the table which contains a node's parent id. If the node is a root node that column will have a null value (or nil). With that, the insertion and removal of nodes is a simple process and thus it should be simple to recover a tree in a corrupted state and guarantee data consistency.

Also queries that traverse the tree (such as get ancestors or descendants of a node) are made in one single recursive SQL statement.

Add this line to your application's Gemfile:

gem 'xylem', '~> 0.1.1'

And then execute:

$ bundle

Now add a parent_id column to your ActiveRecord::Base model. Let's suppose that you want to add the tree behavior to a model called Menu with a database table called menus.

You could invoke the rails generator tool like this:

rails g migration add_parent_to_menus parent:references

Or you could create the migration by hand, with something like that:

class AddParentToMenus < ActiveRecord::Migration
  def change
    add_reference :menus, :parent, index: true, foreign_key: true
  end
end

Some notes regarding migration:

• Add an index is optional, but recommended for performance sake.
• Add a foreign key is optional, but it's recommended to guarantee data consistency
• The foreign_key: true option is available in rails 4.2.1 or newer

Now you need to enable the tree behavior in our model by adding the acts_as_tree directive in it. Let's continue supposing that you're dealing with a model called Menu, you should add the following:

class Menu < ActiveRecord::Base
  acts_as_tree
end

And you are ready to go! Check the config options in the Usage section below.

• :counter_cache => The name of the column that will cache the node children count. In order to use this, you need to create an integer column with the same name (ex: counter_cache: :children_count).
• :touch => If true, when you updated or destroy a node, it's ancestors will be touched: updated_at column is updated with the current time. (the default value is false)
• :dependent => Controls what happens with the children of a deleted node. Choose one of the following options (the default value is :destroy)
  • :destroy children are also destroyed.
  • :delete_all delete children direct in the database (this will skip callbacks)
  • :nullify set the children's parent_id to NULL (nil), therefore turning them into new roots (this will also skip callbacks)
  • :restrict_with_exception an exception is raised if there is an attempt to destroy a record with children
  • :restrict_with_error a validation error is added to the record if there is an attempt to destroy it and it has children

Xylem adds the following class methods to a class with the acts_as_tree directive:

• root: returns the first root of the tree
• roots: returns all the roots from the tree
• leaves: returns all the leaves from the tree

Xylem adds the following class methods to a class with the acts_as_tree directive:

• ancestors: returns the ancestors from root to the parent of the node. Ordered from the root to the parent.
• self_and_ancestors returns the ancestors from the root to the node itself. Ordered from the root to the node.
• descendants returns all the descendants of the given node. Ordered by depth from the closer to the node to the more distant ones.
• self_and_descendants returns the node and all it's descendants. Ordered by depth with the node first and the descendants later.
• root returns the current node's root.
• siblings returns the node's siblings, excluding itself.
• self_and_siblings returns the node and it's siblings.
• children returns the direct children of the node.
• self_and_children returns the node and it's direct children.
• parent returns the node parent.
• root? returns true if the current node is a root, false otherwise.
• leaf? returns true if the current node is a leaf, false otherwise.
• leaves: returns the leaves from the current node

class Menu < ActiveRecord::Base
  acts_as_tree
end

root = Menu.create!(name: 'root menu')
child1 = root.children.create!('option 1')
child2 = Menu.create!(name: 'option 2', parent: root)
subchild = Menu.create!(name: 'suboption 1', parent: child2)

Menu.roots                  # => [ root ]
Menu.leaves                 # => [ child1, subchild ]

root.root?                  # => true
child1.leaf?                # => true

child2.self_and_descendants # => [ child2, subchild ]

child1.root                 # => root
child1.self_and_ancestors   # => [ root, child1 ]

child1.self_and_siblings    # => [ child1, child2 ]

child2.leaves               # => [ subchild ]

• ActiveRecord 4+

This gem relies on the Recursive CTE feature which was introduced to SQL in the 1999 revision. So you need a database management system that implements it. This gem is tested against PostgreSQL and SQLite, those started to support the recursive CTE in the following versions:

• PostgreSQL 8.4+
• SQLite 3.8.3+

It seems that MySQL still has no support to it. If you use MySQL you can look for other gems to help you deal with models organized in trees, check the Alternatives section bellow.

This gem is tested against ruby 1.9.3 and newer only.

There are other gems that allow you to treat models like trees, with different approaches. Here are some other gems you could use to achieve the same objective:

• acts_as_tree
• awesome_nested_set
• ancestry

1. Fork it ( https://github.com/oesgalha/xylem/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

• Fixed leaves query to respect the AR's default_scope

• Added the #leaves instance method

• Initial version

Copyright (c) 2015-2016 Oscar Esgalha

MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.