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Adjacency List trees
====================
.. module:: treebeard.al_tree
This is a simple implementation of the traditional Adjacency List Model for
storing trees in relational databases.
In the adjacency list model, every node will have a
":attr:`~AL_Node.parent`" key, that will be NULL for root nodes.
Since ``django-treebeard`` must return trees ordered in a predictable way,
the ordering for models without the :attr:`~AL_Node.node_order_by`
attribute will have an extra attribute that will store the relative
position of a node between it's siblings: :attr:`~AL_Node.sib_order`.
The adjacency list model has the advantage of fast writes at the cost of
slow reads. If you read more than you write, use
:class:`~treebeard.mp_tree.MP_Node` instead.
.. warning::
As with all tree implementations, please be aware of the
:doc:`caveats`.
.. inheritance-diagram:: AL_Node
.. autoclass:: AL_Node
:show-inheritance:
.. warning::
If you need to define your own
:py:class:`~django.db.models.Manager` class,
you'll need to subclass
:py:class:`~AL_NodeManager`.
.. 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.
Example:
.. code-block:: python
node_order_by = ['field1', 'field2', 'field3']
.. attribute:: parent
``ForeignKey`` to itself. This attribute **MUST** be defined in the
subclass (sadly, this isn't inherited correctly from the ABC in
`Django 1.0`). Just copy&paste these lines to your model:
.. code-block:: python
parent = models.ForeignKey('self',
related_name='children_set',
null=True,
db_index=True)
.. attribute:: sib_order
``PositiveIntegerField`` used to store the relative position of a node
between it's siblings. This attribute is mandatory *ONLY* if you don't
set a :attr:`node_order_by` field. You can define it copy&pasting this
line in your model:
.. code-block:: python
sib_order = models.PositiveIntegerField()
Examples:
.. code-block:: python
class AL_TestNode(AL_Node):
parent = models.ForeignKey('self',
related_name='children_set',
null=True,
db_index=True)
sib_order = models.PositiveIntegerField()
desc = models.CharField(max_length=255)
class AL_TestNodeSorted(AL_Node):
parent = models.ForeignKey('self',
related_name='children_set',
null=True,
db_index=True)
node_order_by = ['val1', 'val2', 'desc']
val1 = models.IntegerField()
val2 = models.IntegerField()
desc = models.CharField(max_length=255)
Read the API reference of :class:`treebeard.models.Node` for info on
methods available in this class, or read the following section for methods
with particular arguments or exceptions.
.. automethod:: get_depth
See: :meth:`treebeard.models.Node.get_depth`
.. autoclass:: AL_NodeManager
:show-inheritance:
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