sqlalchemy / file revision

 """illustrates a quick and dirty way to persist an XML document expressed using ElementTree and pickle.

 This is a trivial example using PickleType to marshal/unmarshal the ElementTree 

 document into a binary column.  Compare to explicit.py which stores the individual components of the ElementTree

 structure in distinct rows using two additional mapped entities.  Note that the usage of both

 styles of persistence are identical, as is the structure of the main Document class.

 """

 from sqlalchemy import (create_engine, MetaData, Table, Column, Integer, String,

     PickleType)

 from sqlalchemy.orm import mapper, create_session

 import sys, os

 from xml.etree import ElementTree

 engine = create_engine('sqlite://')

 meta = MetaData(engine)

 # setup a comparator for the PickleType since it's a mutable

 # element.

 def are_elements_equal(x, y):

     return x == y

 # stores a top level record of an XML document.  

 # the "element" column will store the ElementTree document as a BLOB.

 documents = Table('documents', meta,

     Column('document_id', Integer, primary_key=True),

     Column('filename', String(30), unique=True),

     Column('element', PickleType(comparator=are_elements_equal))

 )

 meta.create_all()

 # our document class.  contains a string name,

 # and the ElementTree root element.  

 class Document(object):

     def __init__(self, name, element):

         self.filename = name

         self.element = element

 # setup mapper.

 mapper(Document, documents)

 ###### time to test ! #########

 # get ElementTree document

 filename = os.path.join(os.path.dirname(__file__), "test.xml")

 doc = ElementTree.parse(filename)

 # save to DB

 session = create_session()

 session.add(Document("test.xml", doc))

 session.flush()

 # clear session (to illustrate a full load), restore

 session.expunge_all()

 document = session.query(Document).filter_by(filename="test.xml").first()

 # print

 document.element.write(sys.stdout)