Better nested set

This plugin provides an ehanced acts_as_nested_set mixin for ActiveRecord, the object-db mapping layer of the framework RubyOnRails. The original nested set feature seems to be quite old and missed some necessary functionalities.

Details

This acts provides Nested Set functionality. Nested Set is a smart way to implement an ordered tree, with the added feature that you can select the children and all of their descendents with a single query. The drawback is that insertion or move need some complex sql queries. But everything is done behind the curtains by this module!

Nested sets are appropriate each time you want either an orderd tree (menus, commercial categories) or an efficient way of querying big trees (threaded posts).

See www.dbmsmag.com/9603d06.html for nested sets theory, and a tutorial here: threebit.net/tutorials/nestedset/tutorial1.html

Small nested set theory reminder

Instead of picturing a leaf node structure with children pointing back to their parent, the best way to imagine how this works is to think of the parent entity surrounding all of its children, and its parent surrounding it, etc. Assuming that they are lined up horizontally, we store the left and right boundries in the database.

Imagine:

  root
    |_ Child 1
      |_ Child 1.1
      |_ Child 1.2
    |_ Child 2
      |_ Child 2.1
      |_ Child 2.2

If my cirlces in circles description didn’t make sense, check out this sweet ASCII art:

    ___________________________________________________________________
   |  Root                                                             |
   |    ____________________________    ____________________________   |
   |   |  Child 1                  |   |  Child 2                  |   |
   |   |   __________   _________  |   |   __________   _________  |   |
   |   |  |  C 1.1  |  |  C 1.2 |  |   |  |  C 2.1  |  |  C 2.2 |  |   |
   1   2  3_________4  5________6  7   8  9_________10 11_______12 13  14
   |   |___________________________|   |___________________________|   |
   |___________________________________________________________________|

The numbers represent the left and right boundries. The table then might look like this:

   id | parent_is | left | right | data
    1 |           |    1 |    14 | root
    2 |         1 |    2 |     7 | Child 1
    3 |         2 |    3 |     4 | Child 1.1
    4 |         2 |    5 |     6 | Child 1.2
    5 |         1 |    8 |    13 | Child 2
    6 |         5 |    9 |    10 | Child 2.1
    7 |         5 |   11 |    12 | Child 2.2

So, to get all children of an entry ‘parent’, you

    SELECT * WHERE left IS BETWEEN parent.left AND parent.right

To get the count, it’s (right - left - 1)/2, etc. To get the direct parent, it falls back to using the parent_id field. There are instance methods for all of these.

API

Methods names are aligned on Tree’s ones as much as possible, to make replacment from one by another easier, except for the creation:

in acts_as_tree:

  item.children.create(:name => "child1")

in acts_as_nested_set:

  # adds a new item at the "end" of the tree, i.e. with child.left = max(tree.right)+1
  child = MyClass.new(:name => "child1")
  child.save
  # now move the item to its right place
  child.move_to_child_of my_item

You can use:

move_to_child_of
move_to_right_of
move_to_left_of

and pass them an id or an object.

Other methods added by this mixin are:

root - root item of the tree (the one that has a nil parent; should have left_column = 1 too)
roots - root items, in case of multiple roots (the ones that have a nil parent)
level - number indicating the level, a root being level 0
ancestors - array of all parents, with root as first item
self_and_ancestors - array of all parents and self
siblings - array of all siblings, that are the items sharing the same parent and level
self_and_siblings - array of itself and all siblings
children_count - count of all immediate children
children - array of all immediate childrens
all_children - array of all children and nested children
full_set - array of itself and all children and nested children

These should not be useful, except if you want to write direct SQL:

left_col_name - name of the left column passed on the declaration line
right_col_name - name of the right column passed on the declaration line
parent_col_name - name of the parent column passed on the declaration line

Recommandation

Don‘t name your left and right columns ‘left’ and ‘right’: these names are reserved on most of dbs. Usage is to name them ‘lft’ and ‘rgt’ for instance.

Where to find better_nested_set

This plugin is provided by Jean-Christophe Michel from Symétrie, and is available on

  http://opensource.symetrie.com/trac/better_nested_set/

Subversion repository is on

  http://opensource.symetrie.com/svn/better_nested_set/trunk

License

The 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