...if you're just checking to see if something exists in the database, why
not just try to .load() it, and then construct it afresh if you don't find
it?
This kind of operation is sometimes called an "upsert" ...some database
engines support it, some don't. Most don't. But what all database engines DO
support is a query, followed by either an insert, or an update as
appropriate.
Here's the idiom that should work:
def ensure_object(sess, id):
o = sess.Query(ModelObject).get(id) # if found, o is now loaded into
session
if not o:
o = ModelObject(1, u'title')
sess.save(o)
sess.flush()
return o
On 12/27/07, Denis S. Otkidach <[EMAIL PROTECTED]> wrote:
>
>
> On Dec 26, 2007 10:38 PM, Michael Bayer <[EMAIL PROTECTED]> wrote:
> > if you have an instance which you are unsure if it already exists, you
> > can add it to a session using session.save_or_update(instance). The
> > decision between INSERT and UPDATE is ultimately decided by the
> > presence of an attribute on the instance called "_instance_key".
>
> I'd like mapper to use UPDATE for newly constructed object (i.e.
> object without _instance_key attribute) when a record with the same
> primary key already exists in DB.
>
> > In most cases, this attribute is not something you need to worry about;
> > if an instance has been flushed or loaded from a session, it will have
> > the attribute, or if you've just constructed it and not yet persisted
> > it, the attribute will not be there. If you think you need to
> > manually manipulate this attribute, perhaps you can describe your
> > specific use case so that we can recommend the best way to accomplish
> > it.
>
> OK, below is a use/test case:
>
> --->8---
> import sqlalchemy as sa, logging
> from sqlalchemy.orm import mapper, sessionmaker
>
> logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)
> logging.basicConfig()
>
> class ModelObject(object):
>
> def __init__(self, id, title):
> self.id = id
> self.title = title
>
> metadata = sa.MetaData()
>
> objectTable = sa.Table(
> 'Objects', metadata,
> sa.Column('id', sa.Integer, primary_key=True),
> sa.Column('title', sa.String(255), nullable=False),
> )
>
> objectsMapper = mapper(ModelObject, objectTable)
>
> engine = sa.create_engine('sqlite://')
> metadata.create_all(engine, checkfirst=True)
>
> session = sessionmaker(bind=engine)()
>
> obj = ModelObject(1, u'title')
> session.save(obj)
> session.commit()
>
> session.clear()
>
> # Another program. We have to insure that object with id=1 exists in DB
> and has
> # certain properties.
> obj = ModelObject(1, u'title')
> session.save_or_update(obj)
> session.commit()
> --->8---
>
> >
>
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