Alexis Petrounias <www.petrounias.org>
Any Model can be turned into a tree by inheriting from the abstract CTENode. By default, this model will feature a ForeignKey to self named parent, and with a related_name of children. Furthermore, it will feature a custom CTENodeManager through the CTENode.objects attribute.
Each instance of a CTENode will feature three virtual fields, by default named depth, path, and ordering; these fields are populated through a custom CTE SQL query, and contain, respectively, the depth of a node (starting with root nodes of depth one), the path (an array of primary keys, or an encoded string if the primary key type requires this), and a custom ordering key (usually starting with the DFS or BFS order key, and then extended with custom ordering specified by the concrete Model).
All parameters regarding the configuration of the tree are specified as attributes in the concrete Model inheriting from CTENode, and have the form _cte_node_*. See below for a complete listing and interpretation of each parameter.
An important caveat when using CTE queries is ordering: any CTE query which is cloned and processed through an order_by invocation will result in the CTE ordering of the nodes to be overridden. Therefore, if you wish to maintain proper tree ordering, you must specify any custom fields to order by through the _cte_node_order_by attribute (see below). Unfortunately, it is not possible to re-create tree ordering through an order_by invocation (see technical notes for an explanation).
In case custom ordering among siblings is desired (such as using an integer, or lexicographic order of a string name, and so on), the move method accepts a parameter position which, if not None, is expected to be a callable that is invoked with the destination and the node currently being moved as arguments, before any modification to the parent relationship is made. Thus, the move method delegates to this callable in order to specify or modify order-related attributes, enforce constraints, or even change the attributes of siblings (such as creating a hole in a contiguous integer total order).
cte_forest.models.CTENode
cte_forest.models.CTENodeManager