About this document
This document provides an overview of what Modified Preorder Tree Traversal (MPTT) and Django MPTT are.
MPTT is a technique for storing hierarchical data in a database in a manner which makes tree retrieval operations such as fetching complete trees, item ancestors and item descendants very efficient.
The trade-off for this efficiency is that performing inserts and moving items around the tree is more involved, as there's some extra work required to keep the tree structure in a good state at all times.
Here are a few articles about MPTT to whet your appetite and provide details about how the technique itself works:
In particular, these articles should help you understand the two most important fields in MPTT - the left and right "edge indicators". Other fields involved in Django MPTT's implementation of the technique are discusssed in the technical details documentation.
Django MPTT is a reusable/standalone Django application which aims to make it easy for you to use Modified Preorder Tree Traversal with your own Django models in your own applications.
It takes care of the details of managing a database table as a tree structure and provides tools for working with trees of model instances.
Simple registration of Django model classes for MPTT - fields required specifically for the tree structure will be added dynamically if necessary.
In the simplest case, it's possible to use MPTT with your own models without adding a single line of MPTT-related code to them.
The tree structure is automatically managed when you create or delete MPTT model instances. By default, new instances are added as the last child of their parent if they have one, otherwise they become the root of a new tree.
The tree structure is automatically updated when you change a model instance's parent - by default, modified instances are moved so they are the last child of their new parent.
This allows basic management of the tree using the parent field available to you in the django.contrib.admin application and in newforms forms generated using the ModelForm class.
To override the defaults insertion/reparenting positions mentioned above, you can specify that a particular field in your model is used for ordering when inserting or automatically reparenting model instances.
This allows you to have each level of the tree automatically sorted by a field of your choice.
New methods are added model classes which are registered with MTTP, for changing their position in the tree, retrieving their ancestors, siblings and descendants, determining the number of descendants they have and other tree-related operations.
A custom TreeManager manager is added to all MPTT model classes, providing tree management operations which, among other things, can be used to move any node in a tree (and its descendants) to an arbitrary point elsewhere in the tree, or to insert new nodes at any point in the tree.
newforms components for working with trees in forms.
Utility functions for working with trees of models.
Template tags and filters for working with trees of models in templates.