Adjacency List trees¶
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
parent” key, that will be NULL for root nodes.
django-treebeard must return trees ordered in a predictable way,
the ordering for models without the
attribute will have an extra attribute that will store the relative
position of a node between it’s siblings:
The adjacency list model has the advantage of fast writes at the cost of
slow reads. If you read more than you write, use
As with all tree implementations, please be aware of the Known Caveats.
Abstract model to create your own Adjacency List Trees.
If you need to define your own
Managerclass, you’ll need to subclass
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']
ForeignKeyto 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:
parent = models.ForeignKey('self', related_name='children_set', null=True, db_index=True)
PositiveIntegerFieldused to store the relative position of a node between it’s siblings. This attribute is mandatory ONLY if you don’t set a
node_order_byfield. You can define it copy&pasting this line in your model:
sib_order = models.PositiveIntegerField()
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
treebeard.models.Nodefor info on methods available in this class, or read the following section for methods with particular arguments or exceptions.
Returns: the depth (level) of the node Caches the result in the object itself to help in loops. Parameters: update – Updates the cached value.
Custom manager for nodes in an Adjacency List tree.