Note behavior:
Script:
=============================================
from sqlalchemy import *
from sqlalchemy.orm import *
engine = create_engine('postgres://salespylot:salespy...@localhost:
5444/salespylot',echo=True)
metadata = MetaData()
Session = sessionmaker(bind=engine)
DBSession = Session()
order_table = Table("orders", metadata,
Column("orderid", String, primary_key=True),
Column("customerid", String(255),
ForeignKey('customers.customerid'))
)
class Order(object):
pass
mapper(Order, order_table)
customers_table = Table("customers", metadata,
Column("customerid", String(255), Sequence('customers_id_seq'),
primary_key=True),
Column("phonenumber", String(255)),
Column("firstname", String(255)),
Column("lastname", String(255))
)
class Customer(object):
def __repr__(self):
return '<Customer: name=[%s %s] customerid=[%s]>' %
(self.firstname, self.lastname, self.customerid)
mapper(Customer, customers_table,
properties={'orders': relation(Order,
cascade='refresh-expire,expunge',
single_parent=True,
backref=backref('customer',
cascade='save-update,merge,refresh-
expire,expunge',
lazy=True))})
ord=DBSession.query(Order).get(u'SALE65424')
ord.customerid #this returns the current value, '7'
ord.customerid = u'8' #update this value
ord.customer # lazy load
===========================================================
Output, starting at DBSession.query:
---------------------------------------------------------------
>>> ord=DBSession.query(Order).get(u'SALE65424')
2010-05-05 10:02:07,994 INFO sqlalchemy.engine.base.Engine.0x...2f10
select version()
2010-05-05 10:02:07,997 INFO sqlalchemy.engine.base.Engine.0x...2f10
{}
2010-05-05 10:02:08,001 INFO sqlalchemy.engine.base.Engine.0x...2f10
select current_schema()
2010-05-05 10:02:08,001 INFO sqlalchemy.engine.base.Engine.0x...2f10
{}
2010-05-05 10:02:08,003 INFO sqlalchemy.engine.base.Engine.0x...2f10
BEGIN
2010-05-05 10:02:08,004 INFO sqlalchemy.engine.base.Engine.0x...2f10
SELECT orders.orderid AS orders_orderid, orders.customerid AS
orders_customerid
FROM orders
WHERE orders.orderid = %(param_1)s
2010-05-05 10:02:08,004 INFO sqlalchemy.engine.base.Engine.0x...2f10
{'param_1': u'SALE65424'}
>>>
>>> ord.customerid
u'7'
>>>
>>> ord.customerid = u'8'
>>>
>>> ord.customer
2010-05-05 10:02:08,012 INFO sqlalchemy.engine.base.Engine.0x...2f10
UPDATE orders SET customerid=%(customerid)s WHERE orders.orderid = %
(orders_orderid)s
2010-05-05 10:02:08,012 INFO sqlalchemy.engine.base.Engine.0x...2f10
{'orders_orderid': u'SALE65424', 'customerid': u'8'}
2010-05-05 10:02:08,014 INFO sqlalchemy.engine.base.Engine.0x...2f10
SELECT customers.customerid AS customers_customerid,
customers.phonenumber AS customers_phonenumber, customers.firstname AS
customers_firstname, customers.lastname AS customers_lastname
FROM customers
WHERE customers.customerid = %(param_1)s
2010-05-05 10:02:08,015 INFO sqlalchemy.engine.base.Engine.0x...2f10
{'param_1': u'7'}
<Customer: name=[Veronica Smith] customerid=[7]>
>>>
---------------------------------------------------------------
Even after the join criterion was changed (and even flushed to the
database), the lazy load fetched the 'wrong' customer, using the old
value of the customerid.
Is the behavior by design?
The possible rationale is "this makes relationship loading consistent
regardless of autoflush being enabled and also regardless of eager vs.
lazy loading". But, at the end of the day, the example above just
seems wrong.
A longer term enhancement to sqla might be to automatically expire any
relation whose join columns include a column that gets updated in a
session? Is that a manageable effort?
Kent
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