Hi Michael,
On Fri, 2010-03-26 at 14:30 -0400, Michael Bayer wrote:
> here's the relevant bit of documentation:
>
> http://www.sqlalchemy.org/docs/reference/orm/interfaces.html?highlight=mapperextension#sqlalchemy.orm.interfaces.MapperExtension.before_insert
>
> Column-based attributes can be modified within this method which will
> result in the new value being inserted. However **no** changes to the
> overall flush plan can be made, and manipulation of the Session will not
> have the desired effect. To manipulate the Session within an extension,
> use SessionExtension.
Thanks for the pointer. I read that part before but was not sure if I
have to modify the flush plan.
I attached the modified source code that actually works.
I dislike this solution for the following reasons:
* The extension scans through all new instances which could be quite a
number.
* The session must be modified (okay, no real problem).
* In case multiple classes use the CommonStorage class, the
StorageExtension must be adjusted. It would be better to operate on
CommonStorage instances but I don't know how to find the related classes
before the whole thing goes to the database.
Another question: Any idea when the second SA book will be published? I
bought the Essential SA book but it is a bit outdated covering 0.4.x.
Thanks, Torsten
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import sqlalchemy as sa
import sqlalchemy.orm as orm
import sqlalchemy.orm.interfaces as interfaces
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy
import hashlib
Base = declarative_base()
class CommonStorage(Base):
__tablename__ = "common"
hashval = sa.Column(sa.String, primary_key=True)
value = sa.Column(sa.LargeBinary)
def __init__(self, v):
self.value = v
self.hashval = hashlib.md5(v).hexdigest()
def joinFrom(self, session):
copy = session.query(CommonStorage).filter(self.hashval==CommonStorage.hashval).first()
return copy or self
class StorageUser(Base):
__tablename__ = "user"
id = sa.Column(sa.Integer, primary_key=True)
ref = sa.Column(None, sa.ForeignKey(CommonStorage.hashval))
rel = orm.relation(CommonStorage)
value = association_proxy("rel", "value")
class StorageExtension(interfaces.SessionExtension):
def before_flush(self, session, flush_context, instances=None):
for d in session.new:
if isinstance(d, StorageUser) and d.rel is not None:
original = d.rel
d.rel = d.rel.joinFrom(session)
if original is not d.rel:
session.expunge(original)
engine = sa.create_engine("sqlite:///", echo=True)
Base.metadata.create_all(engine)
session = orm.sessionmaker(bind=engine, extension=StorageExtension(), autoflush=False)()
ua, ub = StorageUser(), StorageUser()
ua.value = ub.value = "Something"
session.add(ua)
session.commit()
session.add(ub)
session.commit()
