Materialized Path trees
.. module:: treebeard.mp_tree
.. moduleauthor:: Gustavo Picon <firstname.lastname@example.org>
This is an efficient implementation of Materialized Path
trees for Django 1.3+, as described by `Vadim Tropashko`_ in `SQL Design
Patterns`_. Materialized Path is probably the fastest way of working with
trees in SQL without the need of extra work in the database, like Oracle's
``CONNECT BY`` or sprocs and triggers for nested intervals.
In a materialized path approach, every node in the tree will have a
:attr:`~MP_Node.path` attribute, where the full path from the root
to the node will be stored. This has the advantage of needing very simple
and fast queries, at the risk of inconsistency because of the
denormalization of ``parent``/``child`` foreign keys. This can be prevented
``django-treebeard`` uses a particular approach: every step in the path has
a fixed width and has no separators. This makes queries predictable and
faster at the cost of using more characters to store a step. To address
this problem, every step number is encoded.
Also, two extra fields are stored in every node:
:attr:`~MP_Node.depth` and :attr:`~MP_Node.numchild`.
This makes the read operations faster, at the cost of a little more
maintenance on tree updates/inserts/deletes. Don't worry, even with these
extra steps, materialized path is more efficient than other approaches.
The materialized path approach makes heavy use of ``LIKE`` in your
database, with clauses like ``WHERE path LIKE '002003%'``. If you think
that ``LIKE`` is too slow, you're right, but in this case the
:attr:`~MP_Node.path` field is indexed in the database, and all
``LIKE`` clauses that don't **start** with a ``%`` character will use
the index. This is what makes the materialized path approach so fast.
Due to a `bug in Django 1.3`_, Materialized Path could be problematic
**ONLY** under the following conditions:
* Proxy models are used. Most users don't use this. If you are not
sure that you are using proxy models, then you are not, and
shouldn't worry about this.
* Django 1.3.X is used
* MySQL is being used.
Solutions to this problem:
* Use PostgreSQL
* Use Django 1.4
.. _`Vadim Tropashko`: http://vadimtropashko.wordpress.com/
.. _`Sql Design Patterns`:
.. _`bug in Django 1.3`: https://code.djangoproject.com/ticket/17918
.. inheritance-diagram:: MP_Node
.. autoclass:: MP_Node
Do not change the values of :attr:`path`, :attr:`depth` or
:attr:`numchild` directly: use one of the included methods instead.
Consider these values *read-only*.
Do not change the values of the :attr:`steplen`, :attr:`alphabet` or
:attr:`node_order_by` after saving your first model. Doing so will
corrupt the tree. If you *must* do it:
1. Backup the tree with :meth:`dump_bulk`
2. Empty your model's table
3. Change :attr:`depth`, :attr:`alphabet` and/or
:attr:`node_order_by` in your model
4. Restore your backup using :meth:`load_bulk` with
``keep_ids=True`` to keep the same primary keys you had.
node_order_by = ['numval', 'strval']
numval = models.IntegerField()
strval = models.CharField(max_length=255)
Read the API reference of :class:`treebeard.Node` for info on methods
available in this class, or read the following section for methods with
particular arguments or exceptions.
.. attribute:: steplen
Attribute that defines the length of each step in the :attr:`path` of
a node. The default value of *4* allows a maximum of
*1679615* children per node. Increase this value if you plan to store
large trees (a ``steplen`` of *5* allows more than *60M* children per
node). Note that increasing this value, while increasing the number of
children per node, will decrease the max :attr:`depth` of the tree (by
default: *63*). To increase the max :attr:`depth`, increase the
max_length attribute of the :attr:`path` field in your model.
.. attribute:: alphabet
Attribute: the alphabet that will be used in base conversions
when encoding the path steps into strings. The default value,
``0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ`` is the most optimal possible
value that is portable between the supported databases (which means:
their default collation will order the :attr:`path` field correctly).
In case you know what you are doing, there is a test that is
disabled by default that can tell you the optimal default alphabet
in your enviroment. To run the test you must enable the
:envvar:`TREEBEARD_TEST_ALPHABET` enviroment variable::
$ TREEBEARD_TEST_ALPHABET=1 python manage.py test treebeard.TestTreeAlphabet
On my Ubuntu 8.04.1 system, these are the optimal values for the
three supported databases in their *default* configuration:
Database Optimal Alphabet
MySQL 5.0.51 0-9A-Z
PostgreSQL 8.2.7 0-9A-Z
.. attribute:: node_order_by
Attribute: a list of model fields that will be used for node
ordering. When enabled, all tree operations will assume this ordering.
node_order_by = ['field1', 'field2', 'field3']
.. attribute:: path
``CharField``, stores the full materialized path for each node. The
default value of it's max_length, *255*, is the max efficient and
portable value for a ``varchar``. Increase it to allow deeper trees (max
depth by default: *63*)
`django-treebeard` uses Django's abstract model inheritance, so:
1. To change the max_length value of the path in your model, you
can't just define it since you'd get a django exception, you have
to modify the already defined attribute::
MyNodeModel._meta.get_field('path').max_length = 1024
2. You can't rely on Django's `auto_now` properties in date fields
for sorting, you'll have to manually set the value before creating
desc = models.CharField(max_length=255)
created = models.DateTimeField(auto_now_add=True)
node_order_by = ['created']
For performance, and if your database allows it, you can safely
define the path column as ASCII (not utf-8/unicode/iso8859-1/etc) to
keep the index smaller (and faster). Also note that some databases
(mysql) have a small index size limit. InnoDB for instance has a
limit of 765 bytes per index, so that would be the limit if your path
is ASCII encoded. If your path column in InnoDB is using unicode,
the index limit will be 255 characters since in MySQL's indexes,
unicode means 3 bytes.
treebeard uses **numconv** for path encoding:
.. attribute:: depth
``PositiveIntegerField``, depth of a node in the tree. A root node
has a depth of *1*.
.. attribute:: numchild
``PositiveIntegerField``, the number of children of the node.
.. automethod:: add_root
.. automethod:: add_child
.. automethod:: add_sibling
.. automethod:: move
.. automethod:: get_tree
This metod returns a queryset.
.. automethod:: find_problems
A node won't appear in more than one list, even when it exhibits
more than one problem. This method stops checking a node when it
finds a problem and continues to the next node.
Problems 1, 2 and 3 can't be solved automatically.
.. automethod:: fix_tree