"""
Illustrates three strategies for persisting and querying XML
documents as represented by ElementTree in a relational
database. The techniques do not apply any mappings to the
ElementTree objects directly, so are compatible with the
native cElementTree as well as lxml, and can be adapted to
suit any kind of DOM representation system. Querying along
xpath-like strings is illustrated as well.

In order of complexity:

* ``pickle.py`` - Quick and dirty, serialize the whole DOM into a BLOB column.  While the example
  is very brief, it has very limited functionality.
* ``adjacency_list.py`` - Each DOM node is stored in an individual table row, with attributes
  represented in a separate table.  The nodes are associated in a hierarchy using an adjacency list
  structure.  A query function is introduced which can search for nodes along any path with a given
  structure of attributes, basically a (very narrow) subset of xpath.
* ``optimized_al.py`` - Uses the same strategy as ``adjacency_list.py``, but associates each
  DOM row with its owning document row, so that a full document of DOM nodes can be
  loaded using O(1) queries - the construction of the "hierarchy" is performed after
  the load in a non-recursive fashion and is much more efficient.

E.g.::

    # parse an XML file and persist in the database
    doc = ElementTree.parse("test.xml")
    session.add(Document(file, doc))
    session.commit()

    # locate documents with a certain path/attribute structure
    for document in find_document('/somefile/header/field2[@attr=foo]'):
        # dump the XML
        print document

"""