Django CTE Trees is an experimental implementation of Adjecency-List trees using
PostgreSQL Common Table Expressions (CTE). Its aim is to completely hide the
management of tree structure, as well as explore the issues related to including
CTE in the Django ORM.

Although handling tree structure in a transparent way is a desirable
characteristic for many applications, the currently known limitations of
including CTE (see below) will be a show-stopper for many other applications.
Unless you know beforehand that these limitations will not affect your
application, this module is not suitable for you, and you should use an
actively managed tree structure (such as
django-mptt <https://github.com/django-mptt/django-mptt/> or
django-treebeard <http://code.tabo.pe/django-treebeard/>).

Characteristics:

- Simple: inheriting from an abstract node model is sufficient to obtain tree
  functionality for any Model.

- Seamless: does not use RawQuerySet, so queries using CTE can be combined with
  normal Django queries, and won't confuse the SQLCompiler or other QuerySets,
  including using multiple databases.

- Self-contained: tree nodes can be manipulated without worrying about
  maintaining tree structure in the database.

- Single query: all tree traversal operations can be performed through a single
  query, including children, siblings, ancestors, roots, and descendants.

- Flexible ordering: supports (a subset of) normal Django ordering as well as
  ordering on tree structure information, including depth and path, in DFS and
  BFS orders.

- Multiple delete semantics: supports Pharaoh, Grandmother, and Monarchy
  deletion patterns.

- Code: unit tests, documentation, comments.

Known limitations:

- Virtual fields not usable in external queries: it is not yet possible to use
  the virtual fields which describe the tree structure (depth, path, and
  ordering information) in queries other than directly on the CTE Nodes.
  Consequently, you cannot order on these fields any Model other than the CTE
  Nodes themselves. See the technical notes for details.

- Cannot merge queries with OR: because CTE relies on custom WHERE clauses added
  through extra, the bitwise OR operator cannot be used with query composition.

- Cannot use new Nodes without loading: immediately after creating a CTE Node,
  it must be read from the database if you need to use its tree structure
  (depth, path, and ordering information).

- Cannot order descending: you cannot order on structure fields (depth, path) or
  additional normal fields combined with structure fields in descending order.
