Being new to sqlalchemy I gave myself an exercise to
test my understanding: duplicating a database through
objects. To my naive understanding, sessions act like
Saran Wrap, they seem to stick to objects when I don't
want them to, and not elsewhere. My toy example:
# An attempt to duplicate a database through objects
from sqlalchemy import *
metadata = DynamicMetaData()
simpleTable = Table('simple', metadata,
Column('simple_id', Integer, primary_key=True),
Column('name', String))
class Simple(object):
def __init__(self, name):
self.name = name
def __repr__(self):
return "%s(%r)" % (self.__class__.__name__, self.name)
mapper(Simple, simpleTable)
# Read from one data base
engine = create_engine('sqlite:///Simple.db')
metadata.connect(engine)
session = create_session(bind_to=engine)
objects = session.query(Simple).select()
# My feeble attempt to divorce the objects from the session
for obj in objects:
session.expunge(obj)
session.close()
# Take a peek to see the objects are still with us
for obj in objects:
print obj,
print ""
# Write to another database
engine = create_engine('sqlite:///SonOfSimple.db')
metadata.connect(engine)
session = create_session(bind_to=engine)
metadata.create_all()
# Connect the objects to the "write" session
for obj in objects:
session.save(obj)
# But here is my sin, apparently my objects were not transient ...
# sqlalchemy.exceptions.InvalidRequestError: Instance
'Simple(u'They')'
# is a detached instance or is already persistent in a different
Session
session.flush()
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