= Closure Tree

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.

See {Bill Karwin}[http://karwin.blogspot.com/]'s excellent
{Models for hierarchical data presentation}[http://www.slideshare.net/billkarwin/models-for-hierarchical-data]
for a description of different tree storage algorithms.

== Setup

Note that closure_tree is being developed for Rails 3.1.0.rc1

1.  Add this to your Gemfile: <code>gem 'closure_tree'</code>

2.  Run <code>bundle install</code>

3.  Add <code>acts_as_tree</code> to your hierarchical model(s).

4.  Add a migration to add a <code>parent_id</code> column to the model you want to act_as_tree.

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

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

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

      class CreateTagHierarchy < ActiveRecord::Migration
        def change
          create_table :tags_hierarchy, :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.
          end

          # For "all progeny of..." selects:
          add_index :tags_hierarchy, [:ancestor_id, :descendant_id], :unique => true

          # For "all ancestors of..." selects
          add_index :tags_hierarchy, [:descendant_id]
        end
      end

6.  Run <code>rake db:migrate</code>

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

    If you're starting from scratch you don't need to call <code>rebuild!</code>.

== Usage

=== Creation

Create a root node:

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

There are two equivalent ways to add children. Either use the <code>add_child</code> method:

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

Or append to the <code>children</code> collection:

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

Then:

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

  child.ancestry_path
  ["grandparent", "parent", "child"]

=== <code>find_or_create_by_path</code>

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

Note that the other columns will be null if nodes are created, other than auto-generated columns like ID and created_at timestamp. Only the specified column will receive the path element value.

== 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.level] returns the level, or "generation", for this node in the tree. A root node = 0
[tag.parent] returns the node's immediate parent
[tag.children] returns an array of immediate children (just those in the next level).
[tag.ancestors] returns an array of all parents, parents' parents, etc, excluding self.
[tag.self_and_ancestors] returns an array of all parents, parents' parents, etc, including self.
[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.

== Thanks to

* https://github.com/collectiveidea/awesome_nested_set
* https://github.com/patshaughnessy/class_factory